Remove documentation of "rejected by plugin" error message

Gerrit no longer reports this error message, so remove it from
the documentation.

Change-Id: I878887bb54712926429a6c9284c9c106e8d1275b
diff --git a/Documentation/error-messages.txt b/Documentation/error-messages.txt
index f8a8cd9..af230e2 100644
--- a/Documentation/error-messages.txt
+++ b/Documentation/error-messages.txt
@@ -35,7 +35,6 @@
 * link:error-squash-commits-first.html[squash commits first]
 * link:error-upload-denied.html[Upload denied for project \'...']
 * link:error-not-allowed-to-upload-merges.html[you are not allowed to upload merges]
-* link:error-prohibited-by-gerrit-reject-by-plugin.html[Prohibited by Gerrit (rejected by plugin X)]
 
 
 General Hints
diff --git a/Documentation/error-prohibited-by-gerrit-rejected-by-plugin.txt b/Documentation/error-prohibited-by-gerrit-rejected-by-plugin.txt
deleted file mode 100644
index 5184b11..0000000
--- a/Documentation/error-prohibited-by-gerrit-rejected-by-plugin.txt
+++ /dev/null
@@ -1,14 +0,0 @@
-Prohibited by Gerrit (rejected by plugin X)
-===========================================
-
-This is the default error message that is returned by Gerrit if a push
-has been rejected because of failed validation by plugin X.
-
-Bear in mind that the (rejected by plugin X) message is always present
-and X indicates the plugin name as it appears in /#/admin/plugins/ list.
-Please refer to the plugin author documentation for knowing the
-validation rule applied.
-
-GERRIT
-------
-Part of link:error-messages.html[Gerrit Error Messages]