fix: remove mergepoint from past migration

[etr2460]

SUMMARY

We try to keep a single branch of all migrations, and this specific migration refers to 2 different down revisions. ae1ed299413b was merged multiple weeks before this migration, and also has a child migration already, so removing it from the chain here should be safe

TESTING INSTRUCTIONS

CI. CI should go through a superset db upgrade and will error if something is broken here

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

to: @betodealmeida @pkdotson @john-bodley
cc: @michellethomas

[betodealmeida]

I think this is safe, since the migrations are not dependent.

[codecov]

Codecov Report

Merging #16301 (46cb35c) into master (ac8e54d) will not change coverage.
The diff coverage is n/a.

❗ Current head 46cb35c differs from pull request most recent head 41c7f9a. Consider uploading reports for the commit 41c7f9a to get more accurate results

@@           Coverage Diff           @@
##           master   #16301   +/-   ##
=======================================
  Coverage   76.43%   76.43%           
=======================================
  Files         999      999           
  Lines       53394    53394           
  Branches     6817     6817           
=======================================
  Hits        40813    40813           
  Misses      12344    12344           
  Partials      237      237           

Flag	Coverage Δ
mysql	81.66% <ø> (+0.04%)	⬆️
postgres	81.69% <ø> (ø)
python	81.77% <ø> (ø)
sqlite	81.33% <ø> (ø)

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