Revert "Fix threading issue in CppUtils::findIncludePaths."

This workaround introduces potential deadlocks with the DUChainLock
which is acquired in many places before calling the findIncludePaths.

I'll have to find another fix for this, for now lets rollback to the
old racy behavior.

This reverts commit 8493390f36027b2d0db33eb30e0f7a42f76f3c34.