MAINT-3161 : Clean up avatar muting code and prototype feature

indra/llwindow/lldragdropwin32.h

indra/newview/app_settings/settings.xml

indra/newview/llavatarrenderinfoaccountant.cpp

indra/newview/llavatarrenderinfoaccountant.h

indra/newview/llviewermenu.cpp

indra/newview/llviewerregion.cpp

indra/newview/llvoavatar.cpp

indra/newview/llvoavatar.h

indra/newview/skins/default/xui/en/menu_attachment_other.xml

indra/newview/skins/default/xui/en/menu_avatar_other.xml