sparc64: Fix prototype warning for vmemmap_free

Fix the following warning:
arch/sparc/mm/init_64.c:2644:6: warning: no previous prototype for ‘vmemmap_free’

The function vmemmap_free() is only used for systems with
CONFIG_MEMORY_HOTPLUG defined - and sparc64 do not support this.
Drop the empty function as it has no users.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: David S. Miller <davem@davemloft.net>
Reviewed-by: Andreas Larsson <andreas@gaisler.com>
Tested-by: Andreas Larsson <andreas@gaisler.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240330-sparc64-warnings-v1-3-37201023ee2f@ravnborg.orgSigned-off-by: Andreas Larsson <andreas@gaisler.com>

arch/sparc/mm/init_64.c

@@ -2640,11 +2640,6 @@ int __meminit vmemmap_populate(unsigned long vstart, unsigned long vend,
 
 	return 0;
 }
-
-void vmemmap_free(unsigned long start, unsigned long end,
-		struct vmem_altmap *altmap)
-{
-}
 #endif /* CONFIG_SPARSEMEM_VMEMMAP */
 
 /* These are actually filled in at boot time by sun4{u,v}_pgprot_init() */