genirq/timings: Remove variance computation code

The variance computation did not provide the expected results and will be
replaced with a different approach to compute the next interrupt based on
the array suffixes derived algorithm.

There is no good way to transform the variance code to the new algorithm,
so for ease of review remove the existing code first.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: rjw@rjwysocki.net
Cc: ulf.hansson@linaro.org
Cc: linux-pm@vger.kernel.org
Link: https://lkml.kernel.org/r/20190328151336.5316-1-daniel.lezcano@linaro.org