iommu: remove stale declaration left over by a merge conflict

The merge commit ae3325f7 ("Merge branches 'arm/smmu', 'mediatek',
's390', 'ti/omap', 'riscv' and 'core' into next") left a stale
declaration of 'iommu_present()' even though the 'core' branch that was
merged had removed the function (and the declaration).

Remove it for real.

Reported-by: Dmitry Safonov <0x7f454c46@gmail.com>
Cc: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>