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.
2 jobs for qt5-remove-default-build in 2 minutes and 11 seconds
Status Job ID Name Coverage
  Test
passed #55691
smoke-test

00:02:11

passed #55690
unit-test

00:00:37