Remove an unused local variable from head.S.  It seems this was never
used even from the initial commit
9703d9d7 (arm64: Kernel booting and
initialisation), and is a left over from a previous implementation
of __calc_phys_offset.
Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>