Remove deprecated ReportEventHandled rpc from piped API

[khanhtc1202]

What this PR does:

as title

Why we need it:

This RPC is no more used by piped(s) from 3 years ago 👴

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 26.57%. Comparing base (874278e) to head (2df728b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5620      +/-   ##
==========================================
+ Coverage   26.56%   26.57%   +0.01%     
==========================================
  Files         475      475              
  Lines       50592    50582      -10     
==========================================
+ Hits        13438    13444       +6     
+ Misses      36091    36075      -16     
  Partials     1063     1063              

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[Warashi]

I found the definitions of request/response messages remain.
Please remove them.

pipecd/pkg/app/server/service/pipedservice/service.proto

Lines 476 to 481 in 771da47

	message ReportEventsHandledRequest {
	repeated string event_ids = 1 [(validate.rules).repeated.min_items = 1];
	}
	message ReportEventsHandledResponse {
	}

[khanhtc1202]

@Warashi Thanks, removed by 2df728b

[Warashi]

Thank you. LGTM.

[t-kikuc]

I confirmed that ReportEventsHandled() is not called anywhere.

What "old Piped" means is unclear 😓