config-schema.json: fixup to default-command

Before this commit, both VS Code's "Even better TOML" and `taplo`
reported an error saying `["status","--no-pager"] is not of type
"string"` for

```toml 
"$schema" = "https://jj-vcs.github.io/jj/latest/config-schema.json"
ui.default-command = ["status", "--no-pager"]
``` 

I believe the schema was either invalid or just ignored `oneOf` because
it was trivially satisfied whenever the `type` restriction was
satisfied.

cli/src/config-schema.json

@@ -64,7 +64,6 @@
                     "default": false
                 },
                 "default-command": {
-                    "type": "string",
                     "description": "Default command to run when no explicit command is given",
                     "default": "log",
                     "oneOf": [