Remove djangorestframework requirement

This conflicts with pretalx's djangorestframework requirement.

Author: martey

requirements/production.txt

@@ -7,7 +7,6 @@ django-constance[redis]==2.9.1
 django-crispy-forms==1.14.0
 django-model-utils==4.2.0
 django-multiselectfield==0.1.12
-djangorestframework==3.13.1
 django-taggit==1.2.0
 django-timezone-field==4.2.3
 easy-thumbnails==2.8.3