Remove sqlalchemy-migrate 0.7.3 patching

Now that sqlalchemy-migrate 0.8.x is available, remove the 0.7.3
patching. This allows environments to pick up 0.8.1 automatically, while
avoiding the version string error. Once an update to global-requirements
is made, another patch will pin our sqlalchemy-migrate version >= 0.8.2.

This is copied from change I5f80dfc07f8256716d025002fc1a1f47e58b2142

Change-Id: I646a5a8f427095e20b09fbf2db796d2644def6fc
Closes-bug: 1251700