Remove package private dead code from the subscriber client library.

[dpcollins-google]

None of this code is ever called, and none of it is usable outside of the library.

[codecov]

Codecov Report

Merging #4589 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@ Coverage Diff @@## master #4589 +/- ##
============================================
- Coverage 49.18% 49.17% -0.01% + Complexity 21962 21952 -10 
============================================
Files 2078 2078 Lines 207507 207457 -50 Branches 24121 24107 -14 ============================================
- Hits 102055 102017 -38 + Misses 97276 97267 -9 + Partials 8176 8173 -3

Impacted Files	Coverage Δ	Complexity Δ
...cloud/pubsub/v1/StreamingSubscriberConnection.java	63.2% <ø> (-7.69%)	9 <0> (-9)
...in/java/com/google/cloud/pubsub/v1/Subscriber.java	83.13% <100%> (+3.89%)	17 <0> (ø)	⬇️
...gle/cloud/pubsub/v1/SubscriptionAdminSettings.java	14.28% <0%> (-1.59%)	4% <0%> (-1%)
...gle/cloud/storage/testing/RemoteStorageHelper.java	57.72% <0%> (+2.43%)	8% <0%> (ø)	⬇️

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