[mlir][Linalg] Drop check for output indexing maps.

The current check for form of the output indexing maps disallows
generic ops that return both a reduced and unreduced value. Such an op
seems like it could fall within the scope of a Strucutred op. Drop the
check. The only load-bearing place this was found to cause isseus was
during vectorization, but the fix for that seems to be simple.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D132716