elixir: Extract to zed-extensions/elixir repository (#26167)

This PR extracts the Elixir extension to the
[zed-extensions/elixir](https://github.com/zed-extensions/elixir)
repository.

Release Notes:

- N/A

Cargo.lock

@@ -16996,13 +16996,6 @@ dependencies = [
  "serde",
 ]
 
-[[package]]
-name = "zed_elixir"
-version = "0.1.4"
-dependencies = [
- "zed_extension_api 0.2.0",
-]
-
 [[package]]
 name = "zed_emmet"
 version = "0.0.3"
@@ -17028,17 +17021,6 @@ dependencies = [
  "wit-bindgen",
 ]
 
-[[package]]
-name = "zed_extension_api"
-version = "0.2.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9fd16b8b30a9dc920fc1678ff852f696b5bdf5b5843bc745a128be0aac29859e"
-dependencies = [
- "serde",
- "serde_json",
- "wit-bindgen",
-]
-
 [[package]]
 name = "zed_extension_api"
 version = "0.3.0"