ref(hooks): remove push hook model as the builder stopped using it

[helgi]

ref deis/builder#407

[deis-bot]

@bacongobbler, @mboersma and @Joshua-Anderson are potential reviewers of this pull request based on my analysis of git blame information. Thanks @helgi!

[Joshua-Anderson]

Is it okay to break backwards compatibility with the hooks api? It might make sense just to make /v2/hooks/push always return a 200 and mark it as deprecated.

[helgi]

The current approach only affects people that do not update the builder but do update the controller - That's only a developer workflow as it stands since we do not support that behaviour outside of dev.

That being said I could keep the route, but people could also just clean install dev :P

[bacongobbler]

LGTM since this does not affect the end user at all. This is explicitly an internal API and is not supported outside of the builder/controller.

[codecov-io]

Current coverage is 86.21% (diff: 100%)

Merging #985 into master will decrease coverage by 0.12%

@@             master       #985   diff @@
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  Files            29         28     -1   
  Lines          3191       3155    -36   
  Methods           0          0          
  Messages          0          0          
  Branches        538        537     -1   
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- Hits           2755       2720    -35   
+ Misses          289        288     -1   
  Partials        147        147          

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