[migrations] Cleanup recent migrations

[john-bodley]

This PR cleans up a couple of issues with some recent migrations:

1. For c5756bec8b47 it removes unnecessary definitions.
2. For 130915240929 it removes the unnecessary merge for each record which significantly impacts performance. The session.commit() is all that is required to commit the mutated tbl objects which is a similar pattern to c5756bec8b47.

to: @michellethomas @mistercrunch

[codecov-io]

Codecov Report

Merging #5155 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5155   +/-   ##
=======================================
  Coverage   77.47%   77.47%           
=======================================
  Files          44       44           
  Lines        8741     8741           
=======================================
  Hits         6772     6772           
  Misses       1969     1969

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[mistercrunch]

LGTM