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

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]