[SimpleLoopUnswitch] Remove duplicate include.

Header "llvm/Transforms/Scalar/SimpleLoopUnswitch.h" is currently
included twice. This commit removes the duplicate 'include' line.

Previous commit 693eedb1
seems to have mistakenly added the duplicate 'include'.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D112979