otelgrpc: Remove code that has no effects in stats handlers

[pellared]

The span returned in TagConn is always noop.
The ctx in TagConn and TagRPC are different.

Related to #4531

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f770d88) 81.1% compared to head (2f1c89a) 81.1%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4773     +/-   ##
=======================================
- Coverage   81.1%   81.1%   -0.1%     
=======================================
  Files        150     150             
  Lines      10759   10753      -6     
=======================================
- Hits        8731    8725      -6     
  Misses      1872    1872             
  Partials     156     156             

Files	Coverage Δ
...n/google.golang.org/grpc/otelgrpc/stats_handler.go	92.8% <100.0%> (-0.3%)	⬇️