Move readthrough delete modal into books dir

bookwyrm/templates/snippets/delete_readthrough_modal.html

@@ -1,25 +0,0 @@
-{% extends 'components/new_modal.html' %}
-{% load i18n %}
-
-{% block modal-title %}{% trans "Delete these read dates?" %}{% endblock %}
-
-{% block modal-body %}
-{% if readthrough.progress_updates|length > 0 %}
-    {% blocktrans trimmed with count=readthrough.progress_updates|length %}
-    You are deleting this readthrough and its {{ count }} associated progress updates.
-    {% endblocktrans %}
-{% else %}
-    {% trans "This action cannot be un-done" %}
-{% endif %}
-{% endblock %}
-
-{% block modal-footer %}
-<form name="delete-readthrough-{{ readthrough.id }}" action="/delete-readthrough" method="POST">
-    {% csrf_token %}
-    <input type="hidden" name="id" value="{{ readthrough.id }}">
-    <button class="button is-danger" type="submit">
-        {% trans "Delete" %}
-    </button>
-    <button type="button" class="button" data-modal-close>{% trans "Cancel" %}</button>
-</form>
-{% endblock %}