PM / devfreq: bw_hwmon: remove unused variable

The bw_hwmon variable undo_over_req_mbps is never
used. Remove it to clean up the code.

Change-Id: Ifb81052c30cdb79d2c9d8dcda56b3cc2a58b8ec7
Signed-off-by: Amir Vajid <avajid@codeaurora.org>