[bitcoin/bitcoin] gui: Drop qt4 support (#13458)

@laanwj pushed 5 commits.

a950ad6 squashme: remove libqt4-dev and xvfb from travis
312bc00 squashme: doc: remove another (Recommended)
6e6b298 squashme: update links in src/qt/README.md
fa43b38 squashme: update link in src/qt/bitcoingui.cpp
2b0ac8a squashme: remove qt4 mention in src/qt/README.md

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