Remove clearEncryptionPassword() from LockPatternUtils

Remove this method which cleared the FDE password, since is no longer
used.  It was only being used by the accessibility settings in the
Settings app, and that caller was removed by http://ag/16624515.

Bug: 208476087
Change-Id: If0c75774555d3503f21857e66cce527c5edfa586
(cherry picked from commit 8e265a9f)
Merged-In: If0c75774555d3503f21857e66cce527c5edfa586