chore(api): remove unused timestamp.proto import

[michaelbeaumont]

Checklist prior to review

• Link to docs PR or issue --
• Link to UI issue or PR --
• Is the issue worked on linked? --
• The PR does not hardcode values that might break projects that depend on kuma (e.g. "kumahq" as a image registry) --
• The PR will work for both Linux and Windows, system specific functions like syscall.Mkfifo have equivalent implementation on the other OS --
• Unit Tests --
• E2E Tests --
• Manual Universal Tests --
• Manual Kubernetes Tests --
• Do you need to update UPGRADE.md? --
• Does it need to be backported according to the backporting policy? --
• Do you need to explicitly set a > Changelog: entry here?

[codecov-commenter]

Codecov Report

Merging #4906 (174bb49) into master (e6abbbe) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4906   +/-   ##
=======================================
  Coverage   46.68%   46.69%           
=======================================
  Files         687      687           
  Lines       46656    46656           
=======================================
+ Hits        21781    21785    +4     
+ Misses      22966    22962    -4     
  Partials     1909     1909           

Impacted Files	Coverage Δ
api/mesh/v1alpha1/kds.pb.go	7.41% <ø> (ø)
pkg/plugins/runtime/gateway/route/sorter.go	66.66% <0.00%> (-5.13%)	⬇️
pkg/gc/finalizer.go	78.26% <0.00%> (+4.34%)	⬆️
pkg/plugins/leader/postgres/leader_elector.go	89.36% <0.00%> (+6.38%)	⬆️

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