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 I646a5a8f427095e20b09fbf2db796d2644def6fc

Closes-bug: 1251700
Change-Id: I51178d5b4dff56b92d2de08a73924bba56e975b6
Merge "Remove sqlalchemy-migrate 0.7.3 patching"