[mlir] Remove getNumberOfExecutions from RegionBranchOpInterface

`getNumRegionInvocations` was originally added for the async reference counting, but turned out to be not useful, and currently is not used anywhere (couldn't find any uses in public github repos). Removing dead code.

Reviewed By: Mogball, mehdi_amini

Differential Revision: https://reviews.llvm.org/D117347