remove a wrong assertion.

* iseq.c (rb_iseq_trace_set): remove a wrong assertion.
  This assertion checked `insn` is `trace_` prefix instruction
  but threaded code `insn` is original code.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60820 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

iseq.c

@@ -2343,7 +2343,6 @@ rb_iseq_trace_set(const rb_iseq_t *iseq, rb_event_flag_t turnon_events)
 	    }
 	}
 	else if (TRACE_INSN_P(insn, iseq_encoded[i])) {
-	    VM_ASSERT(insn - VM_INSTRUCTION_SIZE/2 >= 0);
 	    iseq_encoded[i] = INSN_CODE(insn);
 	}
 	i += insn_len(insn);