hooks: Remove Ruby execution path for postreceive

Zeger-Jan van de Wegrequested to merge
zj-remove-feature-flag-postreceive into master

Currently the only hook in production running their Ruby implementation is the postreceive hook. For GitLab.com the feature flag was toggled, and performs very well. As such the feature will be expose to all GitLab installs, as the feature flag is removed in this change.

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