arm64: remove obsolete selection of MULTI_IRQ_HANDLER

The arm64 config selects MULTI_IRQ_HANDLER, which was renamed to
GENERIC_IRQ_MULTI_HANDLER by commit 4c301f9b

 ("ARM: Convert
to GENERIC_IRQ_MULTI_HANDLER"). The 'new' option is already
selected, so just remove the obsolete entry.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>