Revert "Make credit2 the default scheduler"

Revert "Make credit2 the default scheduler"

The migration code reads scheduler parameters on the sending side and
tries to set them again on the receiving side, failing if this fails;
the result is that a simple upgrade from 4.11 -> 4.12 will now fail
all migrations.  Solving this is not simple; revert the credit2
upgrade until we can sort something out.

This reverts commit ec6662445b1ec9231ca69583863de644206ef313.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>