Revert "futex: workaround migrate_disable/enable in different context"

[ Upstream commit a71221d81cc4873891ae44f3aa02df596079b786 ]

Drop the RT fixup, the futex code will be changed to avoid the need for
the workaround.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>