Remove dead LibraryDevelopment and LocalLibrary build targets

[jasonleenaylor]

Build/LibraryDevelopment.targets lost its only importer when
Build/FieldWorks.proj was deleted, and it still called an
MSBuild.ExtensionPack console task whose UsingTask and package are
both gone. Build/LocalLibrary.targets held a LocalLibrary target that
no target or script invoked, passing a LibpalasoPlatform property
defined nowhere. Manage-LocalLibraries.ps1 replaced both.

Deleted the two .targets files, dropped the LocalLibrary.targets
import from InstallerBuild.proj, and removed the two UseLocalLibraries
reads in PackageRestore.targets, which gated only a Message nothing
could reach. Nothing outside LocalLibrary.targets referenced its items
or properties, so InstallerBuild.proj needs no replacement.

Also deleted DownloadLibraryDependencies.pl, whose only input was the
generated LibraryDevelopment.properties, dropped that file's .gitignore
entry, and pruned the stale references in the Localize.targets error
text and openspec build-phases.md.

Why these were unreachable

Both halves are leftovers from the migration in #789 (90bbc7a), which
replaced the shell half with Build/Manage-LocalLibraries.ps1 and
deleted Build/buildLocalLibraries.sh, but left the MSBuild halves
behind. LibraryDevelopment.targets was orphaned earlier still, when
Build/FieldWorks.proj went away in 5711bf6 (#678), and c2b1ffa
had already removed the UsingTask for the console task that lines 14
and 18 still call. UseLocalLibraries is set by nothing today — no CI
workflow, script, or doc passes /p:UseLocalLibraries=Y, so the one
path that could still reach the dead branch does not exist.

PalasoArtifactsDir / ChorusArtifactsDir / LcmArtifactsDir remain
live and untouched: their defaults in PackageRestore.targets and
Directory.Build.props and all seven consumers are unchanged. Only their
UseLocalLibraries-gated paths are gone. build.ps1 -UseLocalLcm and
scripts/Agent/Copy-LocalLcm.ps1 are also untouched — that is a fast
post-restore DLL overlay, complementary to Manage-LocalLibraries.ps1
rather than redundant with it.

Validation

• .\build.ps1 -CommentHygiene — clean full native + managed build,
  0 warnings, 0 errors.
• .\test.ps1 -CommentHygiene — exit 0, all 5897 tests passed.
• An MSBuild /pp preprocess of InstallerBuild.proj resolves
  BuildInstaller with zero references to PalasoFileNamesForLL,
  ChorusDeps, ConfigName, LiblcmPlatform, or ChorusPlatform.
• BuildWindowsXslAssemblies and VerifyLcmCloned both execute from
  InstallerBuild.proj after the import removal.
• The Release installer build reaches Installer.legacy.targets and
  fails at buildBaseInstaller.bat (exit 9009). Restoring all three
  files and the import reproduces that failure byte-for-byte at the
  same line, so it is pre-existing and unrelated.

---

This change is 

[github-actions]

NUnit Tests

1 files ±0 1 suites ±0 12m 50s ⏱️ +22s
6 002 tests ±0 5 921 ✅ ±0 81 💤 ±0 0 ❌ ±0 
6 011 runs ±0 5 930 ✅ ±0 81 💤 ±0 0 ❌ ±0 

Results for commit 277eadc. ± Comparison against base commit f2829d6.

♻️ This comment has been updated with latest results.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.56%. Comparing base (f2829d6) to head (277eadc).

Additional details and impacted files

@@ Coverage Diff @@## main #1113 +/- ##
==========================================
- Coverage 38.56% 38.56% -0.01% 
==========================================
Files 1513 1513 Lines 350995 350995 Branches 40350 40350 ==========================================
- Hits 135351 135348 -3 - Misses 186427 186429 +2 - Partials 29217 29218 +1 

see 4 files with indirect coverage changes

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