Clean identifiers from ycmd database as they're deleted

[abingham]

In the vim ycm client, outdated/deleted identifiers are quickly removed from the ycmd database because it ships code for re-parsing when the user exits insert mode. The result is that stale identifiers are not often used for completion. See #22 for more information.

We should consider techniques for doing this in the emacs client.

[noxdafox]

Same comment in #24