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: 1008a11590b966b4 ("m68k: switch to MEMBLOCK + NO_BOOTMEM")
Signed-off-by: Geert Uytterhoeven <[e-mail]>
Acked-by: Arnd Bergmann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/m68k/sun3/config.c

···
 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;