feat: remove subquery from projection in the grammar

We don't want to mess with subqueries in projection as our query
set doesn't contain such queries. It also simplifies some
transformations (equality propagation for example).

src/frontend_sql/grammar.pest

@@ -7,7 +7,7 @@ Query = _{ Select | UnionAll | Values | Insert }
         (^"on" ~ Condition)? ~ (^"where" ~ Selection)?
     }
         Projection = { Column ~ ("," ~ Column)*? }
-            Column = { Row | Asterisk | Alias | Value | SubQuery }
+            Column = { Row | Asterisk | Alias | Value }
                 Alias = {Value ~ ^"as" ~ (String | QuotedName) }
                 Asterisk = @{ "*" }
         Selection = { Expr* }