posix-cpu-timers: Remove private interval storage

Posix CPU timers store the interval in private storage for historical
reasons (it_interval used to be a non scalar representation on 32bit
systems). This is gone and there is no reason for duplicated storage
anymore.

Use it_interval everywhere.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "H.J. Lu" <hjl.tools@gmail.com>
Link: https://lkml.kernel.org/r/20190111133500.945255655@linutronix.de