Do not regenerate the dictionary file when a model is removed

What does this MR do and why?

Problem:

1. Run bundle exec rails db:migrate
2. Observe the following diff:

$ git diff
diff --git a/db/docs/work_item_types.yml b/db/docs/work_item_types.yml
index 37d2c47de252..445ea9df371d 100644
--- a/db/docs/work_item_types.yml
+++ b/db/docs/work_item_types.yml
@@ -1,7 +1,6 @@
 ---
 table_name: work_item_types
 classes:
-- AddWidgetsForWorkItemTypes::WorkItemType
 - WorkItems::Type
 feature_categories:
 - team_planning
diff --git a/db/docs/work_item_widget_definitions.yml b/db/docs/work_item_widget_definitions.yml
index 59cbca149089..073b0c8cc591 100644
--- a/db/docs/work_item_widget_definitions.yml
+++ b/db/docs/work_item_widget_definitions.yml
@@ -1,7 +1,6 @@
 ---
 table_name: work_item_widget_definitions
 classes:
-- AddWidgetsForWorkItemTypes::WidgetDefinition
 - WorkItems::WidgetDefinition
 feature_categories:
 - team_planning

---

Why:

This happens when the class is added/removed in a separated merge request (not in the same where we have the migration).

---

Solution:

We should not regenerate the dictionary file if a model class is removed from our codebase. We wanna avoid having changed files after the execution of gdk update if someone forgets to update the dictionary classes field. If a new model class is added, we append the new class to the current classes of the dictionary.

Slack thread (internal)

Issue: gitlab-org/database-team/team-tasks#328 (closed)

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