Chore: remove rmcp feature and exp flag usages (#8087)

### Summary
With codesigning on Mac, Windows and Linux, we should be able to safely
remove `features.rmcp_client` and `use_experimental_use_rmcp_client`
check from the codebase now.

codex-rs/core/src/features/legacy.rs

@@ -17,10 +17,6 @@ const ALIASES: &[Alias] = &[
         legacy_key: "experimental_use_unified_exec_tool",
         feature: Feature::UnifiedExec,
     },
-    Alias {
-        legacy_key: "experimental_use_rmcp_client",
-        feature: Feature::RmcpClient,
-    },
     Alias {
         legacy_key: "experimental_use_freeform_apply_patch",
         feature: Feature::ApplyPatchFreeform,
@@ -50,7 +46,6 @@ pub struct LegacyFeatureToggles {
     pub include_apply_patch_tool: Option<bool>,
     pub experimental_use_freeform_apply_patch: Option<bool>,
     pub experimental_use_unified_exec_tool: Option<bool>,
-    pub experimental_use_rmcp_client: Option<bool>,
     pub tools_web_search: Option<bool>,
     pub tools_view_image: Option<bool>,
 }
@@ -75,12 +70,6 @@ impl LegacyFeatureToggles {
             self.experimental_use_unified_exec_tool,
             "experimental_use_unified_exec_tool",
         );
-        set_if_some(
-            features,
-            Feature::RmcpClient,
-            self.experimental_use_rmcp_client,
-            "experimental_use_rmcp_client",
-        );
         set_if_some(
             features,
             Feature::WebSearchRequest,