Remove acron plugin from repo in favour of submodule

Author: asmecher

.gitmodules

@@ -25,3 +25,6 @@
 [submodule "lib/ui-library"]
 	path = lib/ui-library
 	url = https://github.com/pkp/ui-library
+[submodule "plugins/generic/acron"]
+	path = plugins/generic/acron
+	url = https://github.com/pkp/acron