requirements/pip.sh: drop forced pip downgrade

All URL deps should have been updated to work with newer pip versions.

Author: radhermit

requirements/pip.sh

@@ -6,10 +6,6 @@
 DIR=${BASH_SOURCE[0]%/*}
 PACKAGES=( "$@" )
 
-# Force using a older version of pip that allows using URL deps in
-# pyproject.toml build-system.requires.
-pip install -U 'pip<20'
-
 # Try installing the latest build/runtime deps once, if they don't exist
 # install directly from the git.
 INSTALLED="${VIRTUAL_ENV}"/.installed_deps