mm: remove unused stub for can_swapin_thp()

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 6863f5643dd7 ("kbuild: allow Clang to find unused static
inline functions for W=1 build").

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 242d12c98174 ("mm: support large folios swap-in for sync io devices")
Signed-off-by: Andy Shevchenko <[email protected]>
Acked-by: Barry Song <[email protected]>
Cc: Bill Wendling <[email protected]>
Cc: Chuanhua Han <[email protected]>
Cc: Justin Stitt <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>