Remove dependency on promgcstat as this is a private gem

We can load it for CI but for the benchmarking step we push a docker image
which can't be built without adding the token in. As this is a public repo
I'm not sure we want to add it in.

que.gemspec

@@ -26,7 +26,6 @@ Gem::Specification.new do |spec|
   # instead, and in any other clients of `Que`.
   # This is highly non ideal, but unless we properly fork, we have to do this for now.
   spec.add_dependency "prometheus-client"
-  spec.add_dependency "prometheus_gcstat"
 
   spec.add_dependency "rack", ">= 2", "< 4"
   spec.add_dependency "rackup"