mm/page_alloc: fix build with CONFIG_UNACCEPTED_MEMORY=n

mm/page_alloc: fix build with CONFIG_UNACCEPTED_MEMORY=n

When has_unaccepted_memory() is unused, it prevents kernel builds
with clang, `make W=1` and CONFIG_WERROR=y:

mm/page_alloc.c:7036:20: error: unused function 'has_unaccepted_memory' [-Werror,-Wunused-function]
7036 | static inline bool has_unaccepted_memory(void)
|                    ^~~~~~~~~~~~~~~~~~~~~

Fix it by removeing the CONFIG_UNACCEPTED_MEMORY=n stub.

Link: https://lkml.kernel.org/r/20240905142220.49d93337a0abce5690e515d9@linux-foundation.org
Reported-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Closes: https://lkml.kernel.org/r/20240905171553.275054-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>