Remove unused struct field

src/github.com/matrix-org/dendrite/clientapi/auth/storage/accounts/membership_table.go

@@ -56,7 +56,6 @@ const updateMembershipByEventIDSQL = "" +
 type membershipStatements struct {
 	deleteMembershipsByEventIDsStmt  *sql.Stmt
 	insertMembershipStmt             *sql.Stmt
-	selectMembershipByEventIDStmt    *sql.Stmt
 	selectMembershipsByLocalpartStmt *sql.Stmt
 	updateMembershipByEventIDStmt    *sql.Stmt
 }