Remove assignee support from checks plugin.

The assignee functionality is deprecated (doesn't have any support in
Web UI).

Google-Bug-Id: b/267456422
Release-Notes: skip
Change-Id: I8a9a72fe1cfa504933ad0711ea1564103efd9b37

java/com/google/gerrit/plugins/checks/CheckerQuery.java

diff --git a/java/com/google/gerrit/plugins/checks/CheckerQuery.java b/java/com/google/gerrit/plugins/checks/CheckerQuery.java
index 4b7eed4..8cb18f6 100644
--- a/java/com/google/gerrit/plugins/checks/CheckerQuery.java
+++ b/java/com/google/gerrit/plugins/checks/CheckerQuery.java
@@ -88,7 +88,6 @@
           "added",
           "after",
           "age",
-          "assignee",
           "author",
           "before",
           "branch",

resources/Documentation/rest-api-checkers.md

diff --git a/resources/Documentation/rest-api-checkers.md b/resources/Documentation/rest-api-checkers.md
index 06aa276..a33658d 100644
--- a/resources/Documentation/rest-api-checkers.md
+++ b/resources/Documentation/rest-api-checkers.md
@@ -257,7 +257,7 @@
 
 Currently, the set of allowed query operators is:
 
-`added, after, age, assignee, author, before, branch, committer, deleted, delta,
+`added, after, age, author, before, branch, committer, deleted, delta,
 destination, dir, directory, ext, extension, f, file, footer, hashtag, intopic,
 label, onlyextensions, onlyexts, ownerin, path, r, ref, reviewer, reviewerin,
 size, status, submittable, topic, unresolved, wip`