Remove interfacer linter: it's deprecated and not recommended by the author

[Henry Blyth]

mvdan has formally deprecated their interfacer linter so I think it should be removed from gometalinter, or in the very least not enabled by default.

I had a warning from interfacer that wasn't very useful, and it failed the lint task, so I looked into it and found the author has deprecated it and said that it's not a very useful linter:

A tool that suggests interfaces is prone to bad suggestions, so its usefulness in real code is limited. This tool will remain available as a proof of concept, and for others to examine and learn from.

[Daniel Martí]

This is a good idea. Funnily enough, unparam is much more useful and less prone to false positives, but it's not enabled by default.