Remove unmaintained influxdbstore backend

[slimsag]

• This experimental store project, overall, failed.
• Because it has not been maintained it is also causing CI to fail constantly, e.g. travis: Go 1.7+ is the minimum supported version #182
• It does not build currently, and as far as I know nobody uses it (except maybe a quite old Sourcegraph revision, which is not in use today).

Let's just remove it.