mmc: sh_mobile_sdhi: remove obsolete irq_by_name registration

There is no user left in the kernel, so this code can be removed.
(Legacy, non-DT sh_mobile boards have been removed a while ago.) The

diff looks more complicated than it is: The if-block for multiplexed isr
is now the main code path, the rest is removed.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>