test: removed dead code for unrecognised binary operators

test: removed dead code for unrecognised binary operators

the fallback error in binary_operator() was unreachable, since invalid
binary operators are rejected in three_arguments() via binop() which
returns false and prevents entry into binary_operator().

this dead code was unreachable and safe to remove

ref: line 636 where "binary operator expected" is generated.

Signed-off-by: Harry Fellowes <harryfellowes1@gmail.com>
Copyright-paperwork-exempt: Yes