buildenv/win32-static: remove Qt 5 build from build-all.bash.

The current build of Qt 5 takes up an enormous amount of disk space.
We don't yet know why, but it's causing our builders to not have
enough space for two simultaneous buildenvs.

Since we don't use it in any builds yet, don't build it by default.
buildenv/win32-static: update Qt 4 to latest mumble-developers-qt HEAD.