functional_test: code cleanup and minor enhancements

[ahrtr]

1. Cleaned up some useless or dead code;
2. Remove some unnecessary methods.

Signed-off-by: Benjamin Wang [email protected]

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[codecov-commenter]

Codecov Report

Merging #14602 (bbda1da) into main (e24402d) will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #14602      +/-   ##
==========================================
- Coverage   75.65%   75.50%   -0.16%     
==========================================
  Files         457      457              
  Lines       37300    37223      -77     
==========================================
- Hits        28220    28105     -115     
- Misses       7325     7350      +25     
- Partials     1755     1768      +13     

Flag	Coverage Δ
all	75.50% <ø> (-0.16%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/etcdserver/api/v3lock/lock.go	61.53% <0.00%> (-8.47%)	⬇️
client/pkg/v3/tlsutil/tlsutil.go	83.33% <0.00%> (-8.34%)	⬇️
raft/rafttest/node.go	95.00% <0.00%> (-5.00%)	⬇️
api/etcdserverpb/raft_internal_stringer.go	76.78% <0.00%> (-4.94%)	⬇️
client/pkg/v3/testutil/leak.go	62.83% <0.00%> (-4.43%)	⬇️
client/v3/experimental/recipes/priority_queue.go	63.33% <0.00%> (-3.34%)	⬇️
server/etcdserver/api/v3rpc/member.go	93.54% <0.00%> (-3.23%)	⬇️
client/v3/experimental/recipes/double_barrier.go	71.87% <0.00%> (-3.13%)	⬇️
server/proxy/grpcproxy/watch.go	93.64% <0.00%> (-2.90%)	⬇️
client/v3/experimental/recipes/key.go	75.34% <0.00%> (-2.74%)	⬇️
... and 15 more

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[spzala]

Thanks @ahrtr

[ahrtr]

Thanks @spzala for the review