LCM: Don't erase ROTPK in RMA mode

As erasing the ROTPK prevents BL1_2 from booting, which prevents running
RMA-LCS tests in the BL1_2 testsuite (or in the BL1_1 testsuite if they
are both enabled and hence running in BL1_2). Not erasing the ROTPK is
acceptable from a security standpoint as it is not secret information.

Change-Id: I4be463a11cae20ae9bd7b5d364cd038518edfcfe
Signed-off-by: Raef Coles <raef.coles@arm.com>