arm64: Drop do_el0_ia_bp_hardening() & do_sp_pc_abort() declarations

There is a redundant do_sp_pc_abort() declaration in exceptions.h which can
be removed. Also do_el0_ia_bp_hardening() as been already been dropped with
the commit bfe29874 ("arm64: entry-common: don't touch daif before
bp-hardening") and hence does not need a declaration any more. This should
not introduce any functional change.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>