mvcc: remove unnecessary metrics update code

[xiang90]

/cc @yudai

After your PR, we now update metrics in defrag func. So no need to call commit to update metrics.

[gyuho]

return b.defrag()?

[codecov-io]

Codecov Report

Merging #9300 into master will decrease coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #9300      +/-   ##
==========================================
- Coverage   75.55%   75.47%   -0.09%     
==========================================
  Files         365      365              
  Lines       30702    30692      -10     
==========================================
- Hits        23198    23164      -34     
- Misses       5879     5907      +28     
+ Partials     1625     1621       -4

Impacted Files	Coverage Δ
internal/mvcc/backend/batch_tx.go	77.5% <ø> (-1.08%)	⬇️
internal/mvcc/backend/backend.go	80.09% <100%> (+0.55%)	⬆️
client/members.go	58.87% <0%> (-26.62%)	⬇️
proxy/grpcproxy/register.go	69.44% <0%> (-11.12%)	⬇️
etcdserver/v2_server.go	80.76% <0%> (-3.85%)	⬇️
etcdserver/api/v3election/election.go	64.7% <0%> (-2.95%)	⬇️
pkg/netutil/netutil.go	66.66% <0%> (-2.09%)	⬇️
rafthttp/msgappv2_codec.go	71.3% <0%> (-1.74%)	⬇️
clientv3/leasing/kv.go	90.69% <0%> (-0.67%)	⬇️
pkg/transport/listener_tls.go	65.56% <0%> (-0.67%)	⬇️
... and 16 more

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[gyuho]

lgtm. Thanks.