Remove the cookbook-plugin submodule

The cookbook-plugin was made read-only with the intention of replacing
it with a new examples plugin.

Remove the submodule before any futher changes are made in core Gerrit
that will end up causing build problems due to interface changes.

Change-Id: Id8536dc9c3750eecd76bdf61e1d24af92333ce25
(cherry picked from commit ed4d5ecc7c926070652fdb7267576d1c54982a05)
diff --git a/.gitmodules b/.gitmodules
index 6c4d53c..ec8afee 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -3,11 +3,6 @@
 	url = ../plugins/commit-message-length-validator
 	branch = .
 
-[submodule "plugins/cookbook-plugin"]
-	path = plugins/cookbook-plugin
-	url = ../plugins/cookbook-plugin
-	branch = .
-
 [submodule "plugins/download-commands"]
 	path = plugins/download-commands
 	url = ../plugins/download-commands
diff --git a/plugins/cookbook-plugin b/plugins/cookbook-plugin
deleted file mode 160000
index e6d7594..0000000
--- a/plugins/cookbook-plugin
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit e6d7594621d87859a0e6af361cac1fc3173c3588