remove ligo-common from python3 package requirements

Python 3 supports implicit namespace packages, meaning a base package for the ligo namespace is not required.
3 jobs for python3-no-ligo-common in 3 minutes and 9 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #1284628
build-python27

00:00:40

passed #1284629
build-python37

00:02:01

 
  Docs
passed #1284630
docs

00:01:06