mm: remove unused stub for can_swapin_thp()

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

mm/memory.c:4184:20: error: unused function 'can_swapin_thp' [-Werror,-Wunused-function]

Fix this by removing the unused stub.

See also commit 6863f564 ("kbuild: allow Clang to find unused static
inline functions for W=1 build").

Link: https://lkml.kernel.org/r/20241008191329.2332346-1-andriy.shevchenko@linux.intel.com
Fixes: 242d12c9 ("mm: support large folios swap-in for sync io devices")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Barry Song <baohua@kernel.org>
Cc: Bill Wendling <morbo@google.com>
Cc: Chuanhua Han <hanchuanhua@oppo.com>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memory.c

@@ -4181,11 +4181,6 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
 	return __alloc_swap_folio(vmf);
 }
 #else /* !CONFIG_TRANSPARENT_HUGEPAGE */
-static inline bool can_swapin_thp(struct vm_fault *vmf, pte_t *ptep, int nr_pages)
-{
-	return false;
-}
-
 static struct folio *alloc_swap_folio(struct vm_fault *vmf)
 {
 	return __alloc_swap_folio(vmf);