[CODE HEALTH] Remove last unused nostd namespace alias in otlp_populate_attribute_utils

[thc1006]

Summary

Removes the single remaining misc-unused-alias-decls warning in the project: a file-scope namespace nostd = opentelemetry::nostd; declaration at exporters/otlp/src/otlp_populate_attribute_utils.cc:31.

This declaration was deliberately left in place by #4091 to avoid conflicting with the then in-flight #4090 on the same file. Both have now merged (#4091 by marcalff on 2026-05-18, #4090 by lalitb on 2026-06-01), so the conflict surface is gone.

After this PR, the misc-unused-alias-decls warning count for the project is 0 under both all-options-abiv1-preview and all-options-abiv2-preview.

Why the alias is unused

References to nostd:: inside the OPENTELEMETRY_BEGIN_NAMESPACE block resolve via parent-namespace lookup to opentelemetry::nostd without needing the file-scope alias. The alias was redundant from day one; clang-tidy's misc-unused-alias-decls check correctly flagged it.

There are no global-scope references to nostd:: in this file (only the alias declaration itself, which is removed). Removing it does not affect any include or any other code path. Same pattern as the 29 alias removals in #4091.

Ratchet

• all-options-abiv1-preview warning_limit lowered from 385 to 384
• all-options-abiv2-preview warning_limit lowered from 391 to 390

Both presets reported the warning at this site; both ratchets drop by 1.

Verification

Counts measured against artifacts of the most recent successful CI run on main (workflow clang-tidy.yaml, run 26782221265, head 83c135d8):

Preset	Before (count / limit)	After (predicted)
abiv1-preview	385 / 385	384 / 384
abiv2-preview	391 / 391	390 / 390

Local clang-format --dry-run --Werror against the modified source file: clean.

Test plan

• CI clang-tidy job passes both abiv1-preview and abiv2-preview at the new limits
• No behavioral change (declaration removal only, all uses already resolve via namespace nesting)

Part of #2053

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.01%. Comparing base (83c135d) to head (02bc457).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4114   +/-   ##
=======================================
  Coverage   82.01%   82.01%           
=======================================
  Files         385      385           
  Lines       16031    16031           
=======================================
  Hits        13146    13146           
  Misses       2885     2885           

Files with missing lines	Coverage Δ
...xporters/otlp/src/otlp_populate_attribute_utils.cc	92.96% <ø> (ø)

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