Remove old plugin system and docs for it

Adapted from:
3860e7b99f (diff-c949f93d03f44a4217d7a138f9e2e54a)

configure.ac

@@ -221,10 +221,8 @@ AC_ARG_ENABLE([tests],
 AM_CONDITIONAL(ENABLE_TESTS, test x$enable_tests = xyes)
 
 PLUGIN_LIBTOOL_FLAGS="-module -avoid-version"
-LOADER_LIBTOOL_FLAGS="-module -avoid-version"
 
 AC_SUBST(PLUGIN_LIBTOOL_FLAGS)
-AC_SUBST(LOADER_LIBTOOL_FLAGS)
 
 XED_PLUGINS_DATA_DIR="$datadir/xed/plugins"
 AC_SUBST(XED_PLUGINS_DATA_DIR)
@@ -248,8 +246,6 @@ xed/smclient/Makefile
 xed/Makefile
 help/Makefile
 pixmaps/Makefile
-plugin-loaders/Makefile
-plugin-loaders/c/Makefile
 plugins/Makefile
 plugins/changecase/Makefile
 plugins/docinfo/Makefile