Remove strcmp_const_time

This was carried over from Nova and isn't used in Cinder.  Removed
in Nova via commit 08a5066d with message:

"This function was used with deprecated auth and is no longer used
anywhere in the code, so just remove it."

Change-Id: I2b90a1a723265d9d23f89ec03246de8508d591e5
Merge "Remove strcmp_const_time"