[mlir] remove RegionBranchOpInterface from linalg ops

Linalg structure ops do not implement control flow in the way expected
by RegionBranchOpInterface, and the interface implementation isn't
actually used anywhere. The presence of this interface without correct
implementation is confusing for, e.g., dataflow analyses.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D155841