docs: Remove incompatibility with Solaris shell

There doesn't seem to be a need in checking if a directory exists if we
will just rm -rf it once we affirm that it does.  Instead, just blindly
try to delete it.

This fixes PR17541.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192680 91177308-0d34-0410-b5e6-96231b3b80d8