Remove Expr::MatchPattern (#11367)

# Description
Following from #11356, it looks like `Expr::MatchPattern` is no longer
used in any way. This PR removes `Expr::MatchPattern` alongside
`Type::MatchPattern` and `SyntaxShape::MatchPattern`.

# User-Facing Changes
Breaking API change for `nu_protocol`.

crates/nu-parser/src/flatten.rs

@@ -264,10 +264,6 @@ pub fn flatten_expression(
         Expr::Float(_) => {
             vec![(expr.span, FlatShape::Float)]
         }
-        Expr::MatchPattern(pattern) => {
-            // FIXME: do nicer flattening later
-            flatten_pattern(pattern)
-        }
         Expr::MatchBlock(matches) => {
             let mut output = vec![];
 

crates/nu-parser/src/parse_patterns.rs

@@ -1,5 +1,5 @@
 use nu_protocol::{
-    ast::{Expr, Expression, MatchPattern, Pattern},
+    ast::{MatchPattern, Pattern},
     engine::StateWorkingSet,
     ParseError, Span, SyntaxShape, Type, VarId,
 };
@@ -18,19 +18,6 @@ pub fn garbage(span: Span) -> MatchPattern {
     }
 }
 
-pub fn parse_match_pattern(working_set: &mut StateWorkingSet, span: Span) -> Expression {
-    working_set.enter_scope();
-    let output = parse_pattern(working_set, span);
-    working_set.exit_scope();
-
-    Expression {
-        expr: Expr::MatchPattern(Box::new(output)),
-        span,
-        ty: Type::Any,
-        custom_completion: None,
-    }
-}
-
 pub fn parse_pattern(working_set: &mut StateWorkingSet, span: Span) -> MatchPattern {
     let bytes = working_set.get_span_contents(span);
 

crates/nu-parser/src/parser.rs

@@ -2,7 +2,7 @@ use crate::{
     lex::{lex, lex_signature},
     lite_parser::{lite_parse, LiteCommand, LiteElement, LitePipeline},
     parse_mut,
-    parse_patterns::{parse_match_pattern, parse_pattern},
+    parse_patterns::parse_pattern,
     parse_shape_specs::{parse_shape_name, parse_type, ShapeDescriptorUse},
     type_check::{self, math_result_type, type_compatible},
     Token, TokenContents,
@@ -4475,10 +4475,6 @@ pub fn parse_value(
         _ => {}
     }
 
-    if matches!(shape, SyntaxShape::MatchPattern) {
-        return parse_match_pattern(working_set, span);
-    }
-
     match bytes[0] {
         b'$' => return parse_dollar_expr(working_set, span),
         b'(' => return parse_paren_expr(working_set, span, shape),
@@ -4516,7 +4512,6 @@ pub fn parse_value(
         SyntaxShape::GlobPattern => parse_glob_pattern(working_set, span),
         SyntaxShape::String => parse_string(working_set, span),
         SyntaxShape::Binary => parse_binary(working_set, span),
-        SyntaxShape::MatchPattern => parse_match_pattern(working_set, span),
         SyntaxShape::Signature => {
             if bytes.starts_with(b"[") {
                 parse_signature(working_set, span)
@@ -5960,7 +5955,6 @@ pub fn discover_captures_in_expr(
                 discover_captures_in_expr(working_set, expr, seen, seen_blocks, output)?;
             }
         }
-        Expr::MatchPattern(_) => {}
         Expr::MatchBlock(match_block) => {
             for match_ in match_block {
                 discover_captures_in_pattern(&match_.0, seen);