Remove abbreviations for isindex, applet, keygen

[isindex](https://github.com/whatwg/html/pull/1095), [applet](https://github.com/whatwg/html/pull/1399), [keygen](https://github.com/whatwg/html/pull/2221) have been removed from the HTML spec and should be removed as abbreviations.
Note that they all start with proper English words (is, apple, key) so unintentionally completing these (esp. `is`) is not uncommon.

