Remove --flag: bool support (#11541)

# Description
This is a follow up to: #11365

After this pr, `--flag: bool` is no longer allowed.

I think `ParseWarning::Deprecated` is useful when we want to deprecated
something at syntax level, so I just leave it there for now.

# User-Facing Changes
## Before
```
❯ def foo [--b: bool] {}
Error:   × Deprecated: --flag: bool
   ╭─[entry #15:1:1]
 1 │ def foo [--b: bool] {}
   ·               ──┬─
   ·                 ╰── `--flag: bool` is deprecated and will be removed in 0.90. Please use `--flag` instead, more info: https://www.nushell.sh/book/custom_commands.html
   ╰────
```

## After
```
❯ def foo [--b: bool] {}
Error:   × Type annotations are not allowed for boolean switches.
   ╭─[entry #2:1:1]
 1 │ def foo [--b: bool] {}
   ·               ──┬─
   ·                 ╰── Remove the `: bool` type annotation.
   ╰────
```
# Tests + Formatting
Done

crates/nu-command/tests/commands/def.rs

@@ -172,19 +172,6 @@ fn def_default_value_should_restrict_implicit_type() {
  assert!(actual2.err.contains("expected int"));
 }
 
-#[test]
-fn def_boolean_flags() {
- let actual = nu!("def foo [--x: bool] { $x }; foo --x");
- assert!(actual.err.contains("flag missing bool argument"));
- let actual = nu!("def foo [--x: bool = false] { $x }; foo");
- assert_eq!(actual.out, "false");
- let actual = nu!("def foo [--x: bool = false] { $x }; foo --x");
- assert!(actual.err.contains("flag missing bool argument"));
- // boolean flags' default value should be null
- let actual = nu!("def foo [--x: bool] { $x == null }; foo");
- assert_eq!(actual.out, "true");
-}
-
 #[test]
 fn def_wrapped_with_block() {
  let actual = nu!(

crates/nu-parser/src/parser.rs

@@ -18,8 +18,8 @@ use nu_protocol::{
  },
  engine::StateWorkingSet,
  eval_const::eval_constant,
- span, BlockId, DidYouMean, Flag, ParseError, ParseWarning, PositionalArg, Signature, Span,
- Spanned, SyntaxShape, Type, Unit, VarId, ENV_VARIABLE_ID, IN_VARIABLE_ID,
+ span, BlockId, DidYouMean, Flag, ParseError, PositionalArg, Signature, Span, Spanned,
+ SyntaxShape, Type, Unit, VarId, ENV_VARIABLE_ID, IN_VARIABLE_ID,
 };
 
 use crate::parse_keywords::{
@@ -3603,9 +3603,9 @@ pub fn parse_signature_helper(working_set: &mut StateWorkingSet, span: Span) ->
  } => {
  working_set.set_variable_type(var_id.expect("internal error: all custom parameters must have var_ids"), syntax_shape.to_type());
  if syntax_shape == SyntaxShape::Boolean {
- working_set.warning(ParseWarning::DeprecatedWarning(
- "--flag: bool".to_string(),
- "--flag".to_string(),
+ working_set.error(ParseError::LabeledError(
+ "Type annotations are not allowed for boolean switches.".to_string(),
+ "Remove the `: bool` type annotation.".to_string(),
  span,
  ));
  }

src/tests/test_custom_commands.rs

@@ -149,8 +149,7 @@ fn custom_flag2() -> TestResult {
 #[test]
 fn deprecated_boolean_flag() {
  let actual = nu!(r#"def florb [--dry-run: bool, --another-flag] { "aaa" }; florb"#);
- assert!(actual.err.contains("Deprecated"));
- assert_eq!(actual.out, "aaa");
+ assert!(actual.err.contains("not allowed"));
 }
 
 #[test]