Remove the feature flag for GraphQL

As the last step in having GraphQL generally available, we should remove the feature flag that allows disabling it.

At the time of writing:

ag --literal --group --line-number --column --color --color-match 30\;43 --color-path 1\;32 --smart-case --stats -- Feature.enabled\?\(\:graphql .
File: lib/gitlab/graphql.rb
8:7:      Feature.enabled?(:graphql, default_enabled: true)

File: lib/api/helpers/graphql_helpers.rb
10:37:        return fallback.call unless Feature.enabled?(:graphql)