CMake: Remove support for QTDIR environment variable

Using this variable is problematic is the user has several versions of
Qt installed on their system. There is no way to know ahead of time if
the Qt version pointed to by QTDIR matches the toolchain that is being
targeted.

The Qt installation path can still be easily specified if it's not found
by CMake by setting the Qt5_DIR cache variable after the initial
configuration run, so this shouldn't present an usability issue.