libc: Remove sscanf() and timingsafe_bcmp()

sscanf() is unused and it doesn't work, so it doesn't make sense to
keep it.

timingsafe_bcmp() isn't used anywhere.

Change-Id: Ib5d28ff21d0f3ccc36c5c0fb5474b3384105cf80
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>