Revert "cpufreq: make the "scaling_cur_freq" sysfs entry pollable"

This reverts commit ee974af4.

sysfs_notify of scaling_cur_freq is no longer used by userspace
components. Since sysfs_notify contends on a single mutex, it could
add long delay to CPU frequency scaling.

Remove the sysfs_notify() to speed up CPU frequency scaling.

Change-Id: I8203904d26bb49e465acd2307574c84f1453bd24
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>