Make features not import their own plugins

This seems to be unnecessary? The plugins are already listed in the
plugin tags, so there *should* be no need to list them again under
requires... We didn't do this consistently for all plugins anyway and it
apparently hasn't caused any problems.

de.prob2.disprover.feature/feature.xml

@@ -229,7 +229,6 @@ litigation.
 
    <requires>
       <import plugin="de.prob.core" version="9.5.2" match="equivalent"/>
-      <import plugin="de.prob.eventb.disprover.core" version="2.1.2" match="equivalent"/>
       <import plugin="de.prob.ui" version="7.5.2" match="equivalent"/>
       <import plugin="org.eclipse.core.runtime" version="3.2.0" match="compatible"/>
       <import plugin="org.eclipse.osgi" version="3.16.0" match="compatible"/>

de.prob2.feature/feature.xml

@@ -228,9 +228,6 @@ litigation.
    </license>
 
    <requires>
-      <import plugin="de.bmotionstudio.gef.editor" version="5.6.2" match="equivalent"/>
-      <import plugin="de.prob.core" version="9.5.2" match="equivalent"/>
-      <import plugin="de.prob.ui" version="7.5.2" match="equivalent"/>
       <import plugin="org.eclipse.core.databinding" version="1.10.0" match="compatible"/>
       <import plugin="org.eclipse.core.databinding.beans" version="1.7.0" match="compatible"/>
       <import plugin="org.eclipse.core.expressions" version="3.7.0" match="compatible"/>