futex: Remove unused or redundant includes

Since 82af7aca ("Removal of FUTEX_FD"), some includes related to file
operations aren't needed anymore. More investigation around the includes
showed that a lot of includes aren't required for compilation, possible
due to redundant includes. Simplify the code by removing unused
includes.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200702202843.520764-4-andrealmeid@collabora.com