Revert "st/mesa: execute glFlush asynchronously if no image has been imported/exported"

A number of the piglit glx tests use multiple contexts on a single thread, and previously the flush in MakeCurrent() was enforcing the ordering between draws on those different contexts. When that flush was made ASYNC, now there is nothing ordering the draws because we have two (or more) driver threads for a single frontend thread which is using nothing more than glxMakeCurrent() to enforce the ordering.

This reverts commit 057a702a.