Remove old plugin system and docs for it

Adapted from:
3860e7b99f (diff-c949f93d03f44a4217d7a138f9e2e54a)

debian/control

@@ -13,6 +13,7 @@ Build-Depends: autotools-dev,
                libglib2.0-dev,
                libgtk-3-dev,
                libgtksourceview-3.0-dev,
+               libpeas-dev,
                libsm-dev,
                libx11-dev,
                libxml2-dev,

debian/rules

@@ -6,7 +6,6 @@ DHFLAGS=--parallel
 	dh $@ $(DHFLAGS)
 
 override_dh_install:
-	rm -rfv debian/tmp/usr/lib/*/xed/plugin-loaders/*.la
 	rm -rfv debian/tmp/usr/lib/*/xed/plugins/*.la
 	dh_install --fail-missing
 

debian/xed.install

@@ -1,5 +1,4 @@
 usr/bin/
-usr/lib/*/xed/plugin-loaders/*.so
 usr/lib/*/xed/plugins/
 usr/lib/xed/xed-bugreport.sh
 usr/share/applications/