irqchip/mmp: Remove non-DT codepath

Building with "W=1" warns about missing declarations for
two functions in the mmp irqchip driver:

drivers/irqchip/irq-mmp.c:248:13: error: no previous prototype for 'icu_init_irq'
drivers/irqchip/irq-mmp.c:271:13: error: no previous prototype for 'mmp2_init_icu'

The declarations are present in an unused header, but since there is no
caller, it's best to just remove the functions and the header completely,
making the driver DT-only to match the state of the platform.

Fixes: 77acc85c ("ARM: mmp: remove device definitions")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230516200516.554663-2-arnd@kernel.org