[snippy] Fix snippy build

[asi-sc]

This removes unused files that leads to configuration errors

[asi-sc]

This PR fixes configuration issue

CMake Error at cmake/modules/LLVMProcessSources.cmake:116 (message):                                                                         
  Found unknown source file BlockGenPlan.cpp                                                                                                                                                                                                                                              
                                                                                                                                             
  Please update                                                                                                                              
  llvm/tools/llvm-snippy/lib/Generator/CMakeLists.txt

I verified that with this fix configuration and build stages passed. Lit testing passed as well.