Vcs: Dissolve VcsBaseOptionsPage intermediate hierarchy level

It's effectively setting only three properties nowadays, not enough
to justify a case of reuse-by-inheritance anymore.

Change-Id: Ia0aedbd0dc460a70bed580a3402cf928c7221a77
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/git/settingspage.cpp

@@ -128,6 +128,7 @@ SettingsPage::SettingsPage(Core::IVersionControl *control, QObject *parent) :
 {
     setId(VcsBase::Constants::VCS_ID_GIT);
     setDisplayName(SettingsPageWidget::tr("Git"));
+    setCategory(VcsBase::Constants::VCS_SETTINGS_CATEGORY);
     setWidgetFactory([] { return new SettingsPageWidget; });
 }