Delete VTExplain endpoint in favor of vexplain queries

[c-r-dev]

Description

Delete VTExplain endpoint in favor of vexplain queries.

vtexplain endpoint on vtadmin was depricated with #12163

VTAdmin to use VTGate's vexplain #16412 is fixed (merged to main branch with #17508 )

Related Issue(s)

Fixes #12161

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

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[dbussink]

There's still a failure here:

--- FAIL: TestGetSrvVSchemas (0.05s)
    --- PASS: TestGetSrvVSchemas/unauthorized_actor (0.00s)
    --- FAIL: TestGetSrvVSchemas/full_access (0.00s)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.95%. Comparing base (747c308) to head (c37692a).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17706      +/-   ##
==========================================
+ Coverage   67.79%   67.95%   +0.16%     
==========================================
  Files        1587     1585       -2     
  Lines      255829   255020     -809     
==========================================
- Hits       173427   173306     -121     
+ Misses      82402    81714     -688     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[c-r-dev]

There's still a failure here:

--- FAIL: TestGetSrvVSchemas (0.05s)
    --- PASS: TestGetSrvVSchemas/unauthorized_actor (0.00s)
    --- FAIL: TestGetSrvVSchemas/full_access (0.00s)

thanks for helping to check , confirm this is resolved now with c37692a