arm64/head: Drop unnecessary pre-disable-MMU workaround

The Falkor erratum that results in the need for an ISB before clearing
the M bit in SCTLR_ELx only applies to execution at exception level x,
and so the workaround is not needed when disabling the EL1 MMU while
running at EL2.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Marc Zyngier <maz@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20240415075412.2347624-5-ardb+git@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>