infra: Remove broken discovery sites

Discovery sites are used by the p2 installation system to discover
additional update sites containing eclipse plugins and features. Adding
arbitrary websites as discovery sites is an error and will lead to the
Eclipse update mechanism trying to interpret those websites as update
sites, failing, and then adding error messages in the log on each
update.

Author: Bananeweizen

eclipsecs-sevntu-plugin-feature/feature.xml

@@ -15,8 +15,6 @@ Eclipse Public License - Version 1.0
    </license>
 
    <url>
-      <discovery label="Sources on GitHub" url="https://github.com/sevntu-checkstyle/sevntu.checkstyle"/>
-      <discovery label="SevNTU checks site" url="http://sevntu-checkstyle.github.io/sevntu.checkstyle/"/>
       <discovery label="Eclipse Update site" url="http://sevntu-checkstyle.github.io/sevntu.checkstyle/update-site/"/>
    </url>