remove pdf/png code from admin app entirely

while PDFs work on paas, they only do that because it turns out the
python buildpack happens to have imagemagick preinstalled - if that
ever changes then it'd break. so move those to the template preview
service. This also means we can get rid of weazyprint and wand
dependencies

app/utils.py

@@ -16,8 +16,6 @@ from flask import (
 )
 from flask_login import current_user
 
-from wand.image import Image
-
 from notifications_utils.template import (
     SMSPreviewTemplate,
     EmailPreviewTemplate,
@@ -327,21 +325,6 @@ def get_template(
             )
 
 
-def png_from_pdf(pdf_endpoint):
-    output = BytesIO()
-    with Image(
-        blob=pdf_endpoint.get_data(),
-        resolution=150,
-    ) as image:
-        with image.convert('png') as converted:
-            converted.save(file=output)
-    output.seek(0)
-    return dict(
-        filename_or_fp=output,
-        mimetype='image/png',
-    )
-
-
 def get_current_financial_year():
     now = datetime.utcnow()
     current_month = int(now.strftime('%-m'))