qt5: Remove from default build (kdesrc-build-setup and --initial-setup).
With the advent of Qt6 development, the pace of Qt5 has slowed enough that we can now rely on distribution packages being available and up to date for Plasma development.
Since Qt5 remains a significant pain point for kdesrc-build users, we shouldn't inflict this requirement by default for new users.
See also #31 (closed).