cleanup subtree_config.xml from sockpp

Author: w0lek

dev/subtree_config.xml

@@ -64,9 +64,4 @@
         internal_path="parmys"
         external_url="https://github.com/CAS-Atlantic/parmys-plugin.git"
         default_external_ref="v3.0"/>
-    <subtree
-        name="sockpp"
-        internal_path="libs/EXTERNAL/sockpp"
-        external_url="https://github.com/fpagliughi/sockpp.git"
-        default_external_ref="v1.0.0"/>
 </subtrees>