arm64: Remove stray ARCH_HAS_OPP reference

A reference to ARCH_HAS_OPP was added in commit 333d17e5 (arm64: add
ARCH_HAS_OPP to allow enabling OPP library) however this symbol is no
longer needed after commit 049d595a (PM / OPP: Make OPP invisible
to users in Kconfig).
Signed-off-by: NMark Brown <broonie@linaro.org>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>

arch/arm64/Kconfig

 config ARM64
 	def_bool y
 	select ARCH_HAS_ATOMIC64_DEC_IF_POSITIVE
-	select ARCH_HAS_OPP
 	select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
 	select ARCH_USE_CMPXCHG_LOCKREF
 	select ARCH_WANT_OPTIONAL_GPIOLIB