bootconfig: merge _xbc_exit() into xbc_exit()

Since commit 87ce9e83ab8b ("memblock, treewide: make memblock_free()
handle late freeing"), both branches of xbc_free_mem() call
memblock_free(), and the early argument has no effect.

memblock_free() also does nothing if addr is NULL, so the check
before the call is redundant.

So remove the argument and the NULL check, and merge _xbc_exit() into
xbc_exit().

No functional change.

Link: https://lore.kernel.org/all/20260908165712.1703439-1-ekffu200098@gmail.com/

Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>