Revert GRPC context changes

[dbussink]

These changes were made assuming we could use grpc.NewClient everywhere. But we had failing test for that. In a specific case, we replace it with grpc.Dial but that's not the same.

If we can't move it all to grpc.NewClient yet, we have to keep the context setup we do as well, or otherwise grpc.Dial could hang indefinitely even if the caller context is cancelled since it gets started with the background context.

Related Issue(s)

Broken #15743

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[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 68.41%. Comparing base (a89b2cb) to head (4736a0e).
Report is 1 commits behind head on main.

Files	Patch %	Lines
go/vt/vtgate/grpcvtgateconn/conn.go	66.66%	2 Missing ⚠️
go/vt/vtgate/fakerpcvtgateconn/conn.go	0.00%	1 Missing ⚠️
go/vtbench/client.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15780      +/-   ##
==========================================
- Coverage   68.43%   68.41%   -0.03%     
==========================================
  Files        1556     1556              
  Lines      195433   195437       +4     
==========================================
- Hits       133743   133700      -43     
- Misses      61690    61737      +47     

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