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-cli/src/syntax_highlight.rs

@@ -329,7 +329,6 @@ fn find_matching_block_end_in_expr(
             Expr::ImportPattern(_) => None,
             Expr::Overlay(_) => None,
             Expr::Signature(_) => None,
-            Expr::MatchPattern(_) => None,
             Expr::MatchBlock(_) => None,
             Expr::Nothing => None,
             Expr::Garbage => None,