Bazel: Don't use assets parameter in polygerrit_plugin rule

All assets parameter does is add its content to the output. We don't
really need it, as we add the assets into static directory of plugin
artifact in genrule anyway.

Given that this plugin is the only caller site that is using assets
parameter, we can kill it now in gerrit core and simplify
polygerrit_plugin rule.

Change-Id: I54d4a910631c9410883b7fd64ba0d88638f9d579

BUILD

diff --git a/BUILD b/BUILD
index 8b11bd5..7e2c5c6 100644
--- a/BUILD
+++ b/BUILD
@@ -15,11 +15,14 @@
 
 genrule2(
     name = "cm-static",
-    srcs = [":codemirror_editor"],
+    srcs = [
+        ":codemirror-element",
+        ":codemirror_editor",
+    ],
     outs = ["cm-static.jar"],
     cmd = " && ".join([
         "mkdir $$TMP/static",
-        "cp -r $(locations :codemirror_editor) $$TMP/static",
+        "cp $(SRCS) $$TMP/static",
         "cd $$TMP",
         "zip -Drq $$ROOT/$@ -g .",
     ]),
@@ -40,7 +43,4 @@
 polygerrit_plugin(
     name = "codemirror_editor",
     app = "gr-editor/gr-editor.js",
-    assets = [
-        ":codemirror-element",
-    ],
 )