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

arch/m68k/sun3/config.c

@@ -109,13 +109,10 @@ static void sun3_halt (void)
 static void __init sun3_bootmem_alloc(unsigned long memory_start,
 				      unsigned long memory_end)
 {
-	unsigned long start_page;
-
 	/* align start/end to page boundaries */
 	memory_start = ((memory_start + (PAGE_SIZE-1)) & PAGE_MASK);
 	memory_end = memory_end & PAGE_MASK;
 
-	start_page = __pa(memory_start) >> PAGE_SHIFT;
 	max_pfn = num_pages = __pa(memory_end) >> PAGE_SHIFT;
 
 	high_memory = (void *)memory_end;