Revert "Properly add QtQuickWidgets dependency"

Looks like FreeBSD doesn't have QtQuickWidgets packaged at all, so let's do the
same without requiring it...

This reverts commit e5405f0ae9.

README.asciidoc

@@ -100,7 +100,6 @@ The following software and libraries are required to run qutebrowser:
 * http://qt.io/[Qt] 5.7.1 or newer (5.10 recommended) with the following modules:
   - QtCore / qtbase
   - QtQuick (part of qtbase in some distributions)
-  - QtQuickWidgets (part of qtbase/QtQuick in some distributions)
   - QtSQL (part of qtbase in some distributions)
   - QtOpenGL
   - QtWebEngine, or

doc/changelog.asciidoc

@@ -55,10 +55,6 @@ Added
 Changed
 ~~~~~~~
 
-- New dependency on the `PyQt5.QtQuickWidgets` module (which was already
-  accidentally introduced in v1.3.2). For most distributions, this is already
-  contained in the existing dependencies - only distributions which have
-  separate packages for PyQt submodules might be affected.
 - The Windows/macOS releases now bundle Qt 5.11.1 which is based on
   Chromium 65.0.3325.151 with security fixes up to Chromium 67.0.3396.87.
 - New short flags for commandline arguments: `-B` and `-T` for `--basedir` and

qutebrowser/misc/earlyinit.py

@@ -233,7 +233,6 @@ def check_libraries():
         'PyQt5.QtQml': _missing_str("PyQt5.QtQml"),
         'PyQt5.QtSql': _missing_str("PyQt5.QtSql"),
         'PyQt5.QtOpenGL': _missing_str("PyQt5.QtOpenGL"),
-        'PyQt5.QtQuickWidgets': _missing_str("PyQt5.QtQuickWidgets"),
     }
     _check_modules(modules)