[CMake] Don't apply Export set to clang tools

I can't apply export to tools without getting some strange CMake spew. The behavior here is a bit unexpected. CMake is complaining about static link dependencies not being in the same export set, which shouldn't matter.

In the short term it is easier to just remove the export set (which was just added in r258209) while I sort this out.

llvm-svn: 258214