benchmark/benchmain: Enable buffer pooling by default

[arjan-bal]

This PR enables buffer pooling in the benchmark test to align with the library's current default configuration.

The benchmark originally disabled buffer pooling because the feature was opt-in when introduced (#5862). Since buffer pooling is now enabled by default (#7356), this change ensures the benchmark accurately measures the performance of gRPC's default behavior.

RELEASE NOTES: N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.91%. Comparing base (ece7397) to head (7e43ea6).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8638      +/-   ##
==========================================
- Coverage   82.17%   81.91%   -0.27%     
==========================================
  Files         415      415              
  Lines       40709    40693      -16     
==========================================
- Hits        33453    33332     -121     
- Misses       5883     5989     +106     
+ Partials     1373     1372       -1     

see 27 files with indirect coverage changes

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