JBIDE-13451 Too many "xmlns:"-proposals when requesting content assist for tag attribute name

Issue is fixed.
Wrong extra non-iconified 'xmnls'-attribute proposals are removed from the <ui:component />, <ui:composition />, <ui:decorate /> and <ui:fragment /> tags.
JUnit Test org.jboss.tools.jst.jsp.test.ca.JstCAOnCustomPrefixesTest is updated due to valudate the issue.