Fix resetting "View Modes" preferences to defaults

Summary:
Removed call to settings.readConfig() in ViewSettingsTab::loadSettings(),
because it overwrites default settings with user settings even if called
from ViewSettingsTab::restoreDefaultSettings().

Test Plan:
Settings -> Configure Dolphin -> View Modes:
Choose "Custom Font", "Choose...": other font settings than preselected;
alter "Icon Size" slider positions;
click OK; OK;
note modified text font next to icons, icon sizes

Settings -> Configure Dolphin -> View Modes:
Click "Defaults"
check "Font" is reset to "System Font";
check "Icon Size" slider positions are reset to default positions;
click OK
check text font next to icons, icon sizes are reset to defaults

Subscribers: ltoscano, ngraham, elvisangelaccio, michaelh, Dolphin

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D10190