cmake: Remove MAXPATHLEN define as autoconf does not provide it

Presumably it was added to the CMake system when MAXPATHLEN was still
used by code built for Windows. Currently only lib/Support/Path.inc uses
MAXPATHLEN, and it should be available on all Unices.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223139 91177308-0d34-0410-b5e6-96231b3b80d8