doc: remove readme-qt.md

Now that autotools is used as a build system there is nothing
special about building the GUI anymore. Merge the salvageable
parts into build-unix.md
doc: build with qt5 is currently not possible

Add as a separate commit so that it can be easily reverted once
Qt5 is supported again.
Merge pull request #3092 from laanwj/2013_10_remove_readme_qt

doc: remove readme-qt.md