schemadiff: remove ForeignKeyLoopError and loop detection logic

[shlomi-noach]

Description

Followup to #15431 and #15430, this PR removes altogether all loop/cycle analysis logic and removes ForeignKeyLoopError.

The rationale is that schemadiff should support anything MySQL supports. Some foreign key loop scenarios are unsupported by Query Serving, but the Query Serving logic is more intricate and should not be applied nor duplicated by schemadiff.

Related Issue(s)

#15430

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• 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]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.65%. Comparing base (696fe0e) to head (5c93fff).
Report is 117 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15507      +/-   ##
==========================================
- Coverage   67.41%   65.65%   -1.77%     
==========================================
  Files        1560     1563       +3     
  Lines      192752   194421    +1669     
==========================================
- Hits       129952   127638    -2314     
- Misses      62800    66783    +3983     

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