Gopkg.toml: remove unnecessary constraints and add pruning

[tamird]

Confirmed that dep ensure results in no version changes in Gopkg.lock.

@gyuho

[codecov-io]

Codecov Report

Merging #9258 into master will increase coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9258      +/-   ##
==========================================
+ Coverage   75.74%   75.87%   +0.12%     
==========================================
  Files         363      363              
  Lines       30200    30200              
==========================================
+ Hits        22876    22914      +38     
+ Misses       5718     5683      -35     
+ Partials     1606     1603       -3

Impacted Files	Coverage Δ
pkg/transport/timeout_conn.go	80% <0%> (-20%)	⬇️
pkg/testutil/recorder.go	77.77% <0%> (-3.71%)	⬇️
clientv3/leasing/txn.go	88.09% <0%> (-3.18%)	⬇️
clientv3/watch.go	95.83% <0%> (-0.74%)	⬇️
clientv3/leasing/kv.go	89.7% <0%> (-0.67%)	⬇️
clientv3/health_balancer.go	85.67% <0%> (-0.31%)	⬇️
etcdserver/v3_server.go	81.65% <0%> (+0.25%)	⬆️
etcdserver/server.go	79.51% <0%> (+0.34%)	⬆️
pkg/netutil/netutil.go	68.75% <0%> (+1.04%)	⬆️
internal/lease/leasehttp/http.go	65.44% <0%> (+1.47%)	⬆️
... and 8 more

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[gyuho]

lgtm. Thanks!

/cc @fanminshi and #9224