refactor(ast): Removed redundant type ast.Expr

[asmyasnikov]

specified only from SQLite parser as base (embedded) type for more Expr* types, no usages as standalone type

parser.ExprContext used as embedded type such as

sqlc/internal/engine/sqlite/parser/sqlite_parser.go

Lines 12337 to 12339 in 2e0435c

	type Expr_functionContext struct {
	ExprContext
	}

sqlc/internal/engine/sqlite/parser/sqlite_parser.go

Lines 13340 to 13342 in 2e0435c

	type Expr_qualified_column_nameContext struct {
	ExprContext
	}

sqlc/internal/engine/sqlite/parser/sqlite_parser.go

Lines 12480 to 12482 in 2e0435c

	type Expr_comparisonContext struct {
	ExprContext
	}

etc