Remove PLUGINS from pelicanconf.py

The pelican-webassets plugin is now configured automatically and explicitly
listing it as a plugin causes the build to fail.

Author: tonyroberts

pelicanconf.py

@@ -39,10 +39,6 @@
     'extra'
 ]
 
-PLUGINS = [
-    'pelican_webassets'
-]
-
 EXTRA_PATH_METADATA = {
     'extra/CNAME': {'path': 'CNAME'}
 }