cli: drop support for legacy graph style

I don't think anyone uses the legacy graph style. It's very similar to
the "ascii" style from Sapling's `renderdag` crate.

CHANGELOG.md

@@ -19,6 +19,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   reindexing failed, you'll need to clean up corrupted operation history by
   `jj op abandon ..<bad operation ID>`.
 
+* Dropped support for the "legacy" graph-drawing style. Use "ascii" for a very
+  similar result.
+
 ### New features
 
 * Templates now support logical operators: `||`, `&&`, `!`

cli/src/config-schema.json

@@ -105,7 +105,6 @@
                         "style": {
                             "description": "Style of connectors/markings used to render the graph. See https://github.com/martinvonz/jj/blob/main/docs/config.md#graph-style",
                             "enum": [
-                                "legacy",
                                 "curved",
                                 "square",
                                 "ascii",