gui: Drop qt4 support

Summary:
build: Build system changes to support only Qt5

gui: Remove QT_VERSION fallbacks for Qt < 5

There were surprisingly many #ifdef fallbacks for Qt 4.

Remiving them simplifies maintenance, as well as adding new GUI
functionality.

doc: Remove mention of Qt4 from build docs

This is a backport of core's PR13458

Depends on D1802 and D1818

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1819