Cleanup unused variable.

This patch fixes problem raised in post-review comments of the
https://reviews.llvm.org/D65285. Developers of ittnotify confirmed
that dll_path_ptr field of the __itt_global structure is never used
by ittnotify library, so it is safe to remove the dll_path array.

Differential Revision: https://reviews.llvm.org/D65885

llvm-svn: 368559