Remove the deprecated otelgrpc.StreamClientInterceptor

[MrAlias]

Fix #7105

Indirect clean-up included:

• There are no more tests in interceptor_test.go. The file is deleted, and all helper functions are moved to the unique locations they are now used.
• Unused declarations were removed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.7%. Comparing base (31a4771) to head (ca9495b).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7646     +/-   ##
=======================================
- Coverage   78.7%   78.7%   -0.1%     
=======================================
  Files        186     186             
  Lines      14805   14679    -126     
=======================================
- Hits       11664   11556    -108     
+ Misses      2795    2782     -13     
+ Partials     346     341      -5     

Files with missing lines	Coverage Δ
...ion/google.golang.org/grpc/otelgrpc/interceptor.go	65.0% <ø> (-15.2%)	⬇️

... and 1 file with indirect coverage changes

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