R1:ee2bc07c9025

No-op old search indexing migrations which no longer run and have been obsoleted by upgrade "activities"

Summary:
See T13253. After D20200 (which changed the task schema) these migrations no longer run, since the PHP code will expect a column to exist that won't exist until a `20190220.` migration runs.

We don't need these migrations, since anyone upgrading through September 2017 gets a "rebuild search indexes" activity anyway (see T11932). Just no-op them.

Test Plan: Grepped for…