[OpenMP] Remove the unused testsuite/ directory

[OpenMP] Remove the unused testsuite/ directory

The testsuite directory is not used or updated and confuses new users to the
OpenMP project. These tests were rewritten using the lit format and put under
the runtime/test directory. This patch removes the entire testsuite/ directory.

Differential Revision: https://reviews.llvm.org/D39767

llvm-svn: 318056