Remove ACP feature flags (#38055)

This removes the `gemini-and-native` and `claude-code` feature flags.
Also, I removed a bunch of unused agent1 code that we do not need
anymore.

Initially I wanted to remove much more of the `agent` code, but noticed
some things that we need to figure out first:
- The inline assistant + context strip use `Thread`/`ContextStore`
directly
- We need some replacement for `ToolWorkingSet`, so we can access
available tools (as well as context server tools) in other places, e.g.
the agent configuration and the configure profile modal

Release Notes:

- N/A

crates/feature_flags/src/flags.rs

@@ -29,28 +29,3 @@ pub struct JjUiFeatureFlag {}
 impl FeatureFlag for JjUiFeatureFlag {
     const NAME: &'static str = "jj-ui";
 }
-
-pub struct GeminiAndNativeFeatureFlag;
-
-impl FeatureFlag for GeminiAndNativeFeatureFlag {
-    // This was previously called "acp".
-    //
-    // We renamed it because existing builds used it to enable the Claude Code
-    // integration too, and we'd like to turn Gemini/Native on in new builds
-    // without enabling Claude Code in old builds.
-    const NAME: &'static str = "gemini-and-native";
-
-    fn enabled_for_all() -> bool {
-        true
-    }
-}
-
-pub struct ClaudeCodeFeatureFlag;
-
-impl FeatureFlag for ClaudeCodeFeatureFlag {
-    const NAME: &'static str = "claude-code";
-
-    fn enabled_for_all() -> bool {
-        true
-    }
-}