Gopkg.*: remove directive dependencies from Gopkg.toml

[gyuho]

Reproducible builds are already done via Gopkg.lock.
Remove "constraint"s for external projects with conflicting
dependencies.

See related discussion #9155 (comment).

/cc @tamird @sdboyer @xiang90 @fanminshi

#9224

[gyuho]

@xiang90

Our preferred dependency update workflow was:

1. Update Gopkg.toml
2. Run dep ensure

Similar to what we had with glide.

But the conflicting rules in Gopkg.toml breaks external projects that also use dep.
Now we update dependencies with dep ensure -update package-name-goes-here.

[codecov-io]

Codecov Report

Merging #9240 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9240      +/-   ##
==========================================
- Coverage   75.87%   75.85%   -0.02%     
==========================================
  Files         363      363              
  Lines       30190    30190              
==========================================
- Hits        22907    22901       -6     
- Misses       5684     5685       +1     
- Partials     1599     1604       +5

Impacted Files	Coverage Δ
proxy/grpcproxy/register.go	69.44% <0%> (-13.89%)	⬇️
pkg/testutil/recorder.go	77.77% <0%> (-3.71%)	⬇️
pkg/adt/interval_tree.go	86.48% <0%> (-3.61%)	⬇️
etcdserver/api/v3election/election.go	64.7% <0%> (-2.95%)	⬇️
raft/node.go	90.62% <0%> (-1.79%)	⬇️
rafthttp/msgappv2_codec.go	69.56% <0%> (-1.74%)	⬇️
clientv3/leasing/cache.go	87.77% <0%> (-1.67%)	⬇️
etcdserver/cluster_util.go	76.64% <0%> (-1.46%)	⬇️
clientv3/leasing/kv.go	90.36% <0%> (-1%)	⬇️
etcdserver/server.go	79.63% <0%> (+0.11%)	⬆️
... and 9 more

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[sdboyer]

(didn't mean to comment here, moved to #9224 (comment))