Removal of those variables is done for multiple reasons:
- In general, the caller should set correct variables for compilers,
  this should not be done by the script itself
- In recent macOS versions, default stubs for compilers exist
  (/usr/bin/clang / gcc / cc), so there is not need to use xcrun
  anymore
- Some contribs fail to build with the multi-command notion given by
  xcrun cc, instead they are looking for a tool named "xcrun cc"
  which does not exist