x86/mm: Drop CONFIG_SWIOTLB for PAE

Since kernels with and without CONFIG_X86_PAE are now limited
to the low 4GB of physical address space, there is no need to
use swiotlb any more, so stop selecting this.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250226213714.4040853-7-arnd@kernel.org