Remove newrelic gem dependency

We haven't updated the gem for years and don't know whether it
still works with our current Ruby and Rails versions.

Besides, dependabot keeps opening pull requests to update it. In theory
we could just ignore the dependabot pull requests for this dependency,
but unfortunately right now we can't add a dependabot config file
because it would open pull requests on forks as well.

Finally, there are other companies offering similar services for Rails
applications, and it's up to each CONSUL installation to decide which
one is better for them. We might add a self-hosted performance
monitoring tool in the future.

Since other CONSUL installations might be using Newrelic, and in general
we recommend adding an application monitoring tool, we're suggesting it
in the custom gemfile. In the name of neutrality, we're also adding
Sentry. We might add other services in the future.

Gemfile_custom

@@ -3,3 +3,24 @@
 # * English: https://github.com/consul/consul/blob/master/CUSTOMIZE_EN.md#gemfile
 # * Spanish: https://github.com/consul/consul/blob/master/CUSTOMIZE_ES.md#gemfile
 #
+
+###### Application monitoring services ######
+#
+# By default Consul supports the Errbit self-hosted error monitoring service,
+# through the Airbrake gem. That means Consul also supports using Airbrake as
+# an external service to monitor errors and application performance.
+#
+# Here are some other services which can be used to monitor application errors
+# and performance.
+
+# Newrelic is an external service which helps you monitor the application
+# performance
+# gem "newrelic_rpm", "insert_gem_version_here"
+
+# Sentry is an external or self-hosted service which helps you monitor the
+# application errors and performance
+# gem "sentry-ruby", "insert_gem_version_here"
+
+###### Other gems ######
+#
+# Add your custom gem dependencies here