No ticket: remove selector and context properties

As far as I can tell, the `context` property of jQuery collections exists only to let [jquery-migrate](https://github.com/jquery/jquery-migrate) monkey-patch `.live` and `.die`, and `selector` is completely unnecessary. We should move necessary code into that plugin and cut our undocumented ties with the past.

```
   raw     gz Sizes
271181  80511 dist/jquery.js
 92655  32561 dist/jquery.min.js

   raw     gz Compared to 1.x-master @ 4f786ba4d2a5544cb48f589d2659d6cab84efc34
  -477   -140 dist/jquery.js
  -213    -86 dist/jquery.min.js
```