Remove unused plugin crates (#8350)

This PR removes the unused crates for plugin support.

We're currently exploring Wasm-based extensions, and it's unlikely that
we'll be reusing any of this existing work.

Release Notes:

- N/A

Cargo.lock

@@ -6793,28 +6793,6 @@ dependencies = [
  "time",
 ]
 
-[[package]]
-name = "plugin"
-version = "0.1.0"
-dependencies = [
- "bincode",
- "plugin_macros",
- "serde",
- "serde_derive",
-]
-
-[[package]]
-name = "plugin_macros"
-version = "0.1.0"
-dependencies = [
- "bincode",
- "proc-macro2",
- "quote",
- "serde",
- "serde_derive",
- "syn 1.0.109",
-]
-
 [[package]]
 name = "png"
 version = "0.16.8"