m68k: remove dead code

There are no calls of "watchdog_reset()" anymore anywhere in the tree
since the WATCHDOG_RESET macro got removed in 942d07df ("watchdog:
Remove WATCHDOG_RESET macro").

The only places the identifiers watchdog_disable and watchdog_init are
called are in arch/arm/mach-omap2/, so those can obviously not refer
to these instances.

Hence these functions are not actually used at all and can be
removed. As a bonus, this also removes two leftover references to
WATCHDOG_RESET.

Cc: Huan Wang <alison.wang@nxp.com>
Cc: Angelo Dureghello <angelo@kernel-space.org>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Stefan Roese <sr@denx.de>