[SE] Remove StreamExecutor Summary: The project has been renamed to Acxxel, so this old directory needs to be deleted. Reviewers: jlebar, jprice Subscribers: beanz, mgorny, parallel_libs-commits, modocache Differential Revision: https://reviews.llvm.org/D25964 llvm-svn: 285115 GitOrigin-RevId: b3f709e10f37225ae65c1d48c4623f6abc2cac1e