build: remove now unused UNIX_CAT

THe previous change replaced the use of `cat` or `type` with a custom
python script.  Remove the now unused command determining.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290856 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed