The function qt_commandline_assignment and everything related to it is
removed from configure. It was only used in qtbase, and all usage has
been removed.
More general variable assignments will be added in a subsequent commit.
Task-number: QTBUG-88210
Change-Id: I7cfa782e89914f2b0dc0277c46e425c8a825557e
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>