m68k: mac: Remove unused yday in unmktime()

When building with W=1:

    arch/m68k/mac/misc.c: In function ‘unmktime’:
    arch/m68k/mac/misc.c:557:33: warning: variable ‘yday’ set but not used [-Wunused-but-set-variable]
      557 |         int days, rem, y, wday, yday;
	  |                                 ^~~~

Fix this by removing the unused variable and assignment.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/b25c42eac8beb0862f1b2041e817cb421ec66b4e.1694613528.git.geert@linux-m68k.org