Remove SLF4J Plugin from o.e.help feature

If a Feature includes a Plugin, it usually includes it with a specific
name and the version that was in the TP when the feature was build.
This prevents consumers from using a slf4j bundle with different
symbolic-name or different version in their TP or product.

This is part of
#588

features/org.eclipse.help-feature/feature.xml

@@ -189,13 +189,6 @@
          version="0.0.0"
          unpack="false"/>
 
-   <plugin
-         id="org.slf4j.api"
-         download-size="0"
-         install-size="0"
-         version="0.0.0"
-         unpack="false"/>
-
    <plugin
          id="org.eclipse.core.net"
          download-size="0"