Remove extraneous variation name from OverrideApex

Apexes use an empty variation name for themselves, and only set an
extra variation string for dependencies.  OverrideApex is setting
the variation name for itself, which doesn't match apexes and is
unnecessary.

Test: go test ./apex
Change-Id: I6e726c497ccf10f145cda149bd19849fd22dd20b