Removed experimental "command risk assessment" feature (#7799)

This experimental feature received lukewarm reception during internal
testing. Removing from the code base.

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__approval_modal_exec.snap

@@ -2,6 +2,8 @@
 source: tui/src/chatwidget/tests.rs
 expression: terminal.backend().vt100().screen().contents()
 ---
+
+
   Would you like to run the following command?
 
   Reason: this is a test reason such as one that would be produced by the model

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__exec_approval_history_decision_approved_short.snap

@@ -3,4 +3,3 @@ source: tui/src/chatwidget/tests.rs
 expression: lines_to_single_string(&decision)
 ---
 ✔ You approved codex to run echo hello world this time
-

codex-rs/tui/src/chatwidget/tests.rs

@@ -755,7 +755,6 @@ fn exec_approval_emits_proposed_command_and_decision_history() {
         reason: Some(
             "this is a test reason such as one that would be produced by the model".into(),
         ),
-        risk: None,
         proposed_execpolicy_amendment: None,
         parsed_cmd: vec![],
     };
@@ -800,7 +799,6 @@ fn exec_approval_decision_truncates_multiline_and_long_commands() {
         reason: Some(
             "this is a test reason such as one that would be produced by the model".into(),
         ),
-        risk: None,
         proposed_execpolicy_amendment: None,
         parsed_cmd: vec![],
     };
@@ -851,7 +849,6 @@ fn exec_approval_decision_truncates_multiline_and_long_commands() {
         command: vec!["bash".into(), "-lc".into(), long],
         cwd: std::env::current_dir().unwrap_or_else(|_| PathBuf::from(".")),
         reason: None,
-        risk: None,
         proposed_execpolicy_amendment: None,
         parsed_cmd: vec![],
     };
@@ -2105,7 +2102,6 @@ fn approval_modal_exec_snapshot() {
         reason: Some(
             "this is a test reason such as one that would be produced by the model".into(),
         ),
-        risk: None,
         proposed_execpolicy_amendment: Some(ExecPolicyAmendment::new(vec![
             "echo".into(),
             "hello".into(),
@@ -2157,7 +2153,6 @@ fn approval_modal_exec_without_reason_snapshot() {
         command: vec!["bash".into(), "-lc".into(), "echo hello world".into()],
         cwd: std::env::current_dir().unwrap_or_else(|_| PathBuf::from(".")),
         reason: None,
-        risk: None,
         proposed_execpolicy_amendment: Some(ExecPolicyAmendment::new(vec![
             "echo".into(),
             "hello".into(),
@@ -2376,7 +2371,6 @@ fn status_widget_and_approval_modal_snapshot() {
         reason: Some(
             "this is a test reason such as one that would be produced by the model".into(),
         ),
-        risk: None,
         proposed_execpolicy_amendment: Some(ExecPolicyAmendment::new(vec![
             "echo".into(),
             "hello world".into(),