Remove Sentry reporting for query limiting

Using Sentry, while useful, poses two problems you have to choose from:

1. All errors are reported separately, making it easy to create issues
   but also making it next to impossible to see other errors (due to the
   sheer volume of threshold errors).

2. Errors can be grouped or merged together, reducing the noise. This
   however also means it's (as far as I can tell) much harder to
   automatically create GitLab issues from Sentry for the offending
   controllers.

Since both solutions are terrible I decided to go with a third option:
not using Sentry for this at all. Instead we'll investigate using
Prometheus alerts and Grafana dashboards for this, which has the added
benefit of being able to more accurately measure the behaviour over
time.

Note that throwing errors in test environments is still enabled, and
whitelisting is still necessary to prevent that from happening (and that
in turn still requires that developers create issues).

doc/development/query_count_limits.md

 # Query Count Limits
-Each controller or API endpoint is allowed to execute up to 100 SQL queries. In
-a production environment we'll only log an error in case this threshold is
-exceeded, but in a test environment we'll raise an error instead.
+Each controller or API endpoint is allowed to execute up to 100 SQL queries and
+in test environments we'll raise an error when this threshold is exceeded.
 ## Solving Failing Tests

lib/gitlab/query_limiting/transaction.rb

@@ -51,13 +51,7 @@ module Gitlab
         error = ThresholdExceededError.new(error_message)
-        if raise_error?
-          raise(error)
-        else
-          # Raven automatically logs to the Rails log if disabled, thus we don't
-          # need to manually log anything in case Sentry support is not enabled.
-          Raven.capture_exception(error)
-        end
+        raise(error) if raise_error?
       end
       def increment

spec/lib/gitlab/query_limiting/transaction_spec.rb

@@ -59,18 +59,6 @@ describe Gitlab::QueryLimiting::Transaction do
         expect { transaction.act_upon_results }
           .to raise_error(described_class::ThresholdExceededError)
       end
-
-      it 'reports the error in Sentry if raising an error is disabled' do
-        expect(transaction)
-          .to receive(:raise_error?)
-          .and_return(false)
-
-        expect(Raven)
-          .to receive(:capture_exception)
-          .with(an_instance_of(described_class::ThresholdExceededError))
-
-        transaction.act_upon_results
-      end
     end
   end