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.org


Signed-off-by: Andreas Larsson <andreas@gaisler.com>