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.rs

@@ -69,8 +69,6 @@ pub enum Feature {
     // Experimental
     /// Use the single unified PTY-backed exec tool.
     UnifiedExec,
-    /// Enable experimental RMCP features such as OAuth login.
-    RmcpClient,
     /// Include the freeform apply_patch tool.
     ApplyPatchFreeform,
     /// Allow the model to request web searches.
@@ -226,7 +224,6 @@ impl Features {
         let base_legacy = LegacyFeatureToggles {
             experimental_use_freeform_apply_patch: cfg.experimental_use_freeform_apply_patch,
             experimental_use_unified_exec_tool: cfg.experimental_use_unified_exec_tool,
-            experimental_use_rmcp_client: cfg.experimental_use_rmcp_client,
             tools_web_search: cfg.tools.as_ref().and_then(|t| t.web_search),
             tools_view_image: cfg.tools.as_ref().and_then(|t| t.view_image),
             ..Default::default()
@@ -243,7 +240,6 @@ impl Features {
                 .experimental_use_freeform_apply_patch,
 
             experimental_use_unified_exec_tool: config_profile.experimental_use_unified_exec_tool,
-            experimental_use_rmcp_client: config_profile.experimental_use_rmcp_client,
             tools_web_search: config_profile.tools_web_search,
             tools_view_image: config_profile.tools_view_image,
         };
@@ -348,13 +344,6 @@ pub const FEATURES: &[FeatureSpec] = &[
         },
         default_enabled: false,
     },
-    // Unstable features.
-    FeatureSpec {
-        id: Feature::RmcpClient,
-        key: "rmcp_client",
-        stage: Stage::Experimental,
-        default_enabled: false,
-    },
     FeatureSpec {
         id: Feature::ApplyPatchFreeform,
         key: "apply_patch_freeform",