src: remove unused isolate member · nodejs/node@781713d

Original file line number	Diff line number	Diff line change
`@@ -49,8 +49,7 @@ inline IsolateData::IsolateData(v8::Isolate* isolate, uv_loop_t* event_loop,`
`49`	`49`	`sizeof(StringValue) - 1).ToLocalChecked()),`
`50`	`50`	`PER_ISOLATE_STRING_PROPERTIES(V)`
`51`	`51`	`#undef V`
`52`		`-isolate_(isolate), event_loop_(event_loop),`
`53`		`- zero_fill_field_(zero_fill_field) {}`
	`52`	`+event_loop_(event_loop), zero_fill_field_(zero_fill_field) {}`
`54`	`53`
`55`	`54`	`inline uv_loop_t* IsolateData::event_loop() const {`
`56`	`55`	`return event_loop_;`