m68k: sun3: Remove unused start_page in sun3_bootmem_alloc()

When building with W=1:

    arch/m68k/sun3/config.c: In function ‘sun3_bootmem_alloc’:
    arch/m68k/sun3/config.c:110:23: warning: variable ‘start_page’ set but not used [-Wunused-but-set-variable]
      112 |         unsigned long start_page;
	  |                       ^~~~~~~~~~

Fix this by removing the variable and the assignment, now the last user
is gone.

Fixes: 1008a115 ("m68k: switch to MEMBLOCK + NO_BOOTMEM")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/361c4562350c8739f6c0da34f7c416b2678fae7c.1694613528.git.geert@linux-m68k.org