Remove requirement for id for #markdown_cache_key

It's not needed anymore as we require `#cache_key` instead.

lib/gitlab/markdown_cache/redis/store.rb

@@ -43,9 +43,9 @@ module Gitlab
         end
 
         def markdown_cache_key
-          unless @subject.respond_to?(:id)
+          unless @subject.respond_to?(:cache_key)
             raise Gitlab::MarkdownCache::UnsupportedClassError,
-                  "This class has no id to use for caching"
+                  "This class has no cache_key to use for caching"
           end
 
           "markdown_cache:#{@subject.cache_key}"

spec/lib/gitlab/markdown_cache/redis/extension_spec.rb

@@ -15,10 +15,6 @@ describe Gitlab::MarkdownCache::Redis::Extension, :clean_gitlab_redis_cache do
       cache_markdown_field :title, pipeline: :single_line
       cache_markdown_field :description
 
-      def id
-        "test-markdown-cache"
-      end
-
       def cache_key
         "cache-key"
       end

spec/lib/gitlab/markdown_cache/redis/store_spec.rb

@@ -13,10 +13,6 @@ describe Gitlab::MarkdownCache::Redis::Store, :clean_gitlab_redis_cache do
 
       attr_accessor :field_1, :field_2, :field_1_html, :field_2_html, :cached_markdown_version
 
-      def id
-        'test-redisbacked-store'
-      end
-
       def cache_key
         "cache-key"
       end

spec/models/concerns/cache_markdown_field_spec.rb

@@ -27,10 +27,6 @@ describe CacheMarkdownField, :clean_gitlab_redis_cache do
       cache_markdown_field :title, pipeline: :single_line
       cache_markdown_field :description
 
-      def id
-        "test-markdown-cache"
-      end
-
       def cache_key
         "cache-key"
       end