Remove sort from mergeApexVariations

Remove the sort from mergeApexVariations, and instead sort before
calling it as sorting will break the next change that calls
mergeApexVariations on a provider field that must not be modified.

Also remove the unused ctx PathContext parameter, and use
slices.SortFunc.

Bug: 319288033
Test: Test_mergeApexVariations
Change-Id: I4a044e86a8eb262b54af50afe14c678616c499d1