qt5: Remove the saved superenv pkg-config setup.

 - Qt saves the pkg-config setup from compile time (PKG_CONFIG_LIBDIR). Remove
   this after in `install`. This way pkg-config called from with qmake will be
   controlled by the user's setup.
 - Adresses second half of https://github.com/Homebrew/homebrew/issues/27184
qt5: update 5.2.1 bottle.

Closes #28247.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>