Remove pip-accel from make targets
Remove pip-accel from make targets

pip-accel doesn't properly support installation of editable packages,
which makes splitting out dependencies and setting up test/local
environments more complicated.

It also doesn't appear to be well supported, for example it still
requires pip v7.

Removing pip-accel will likely slow down our test runs for the utils
repo, so if this becomes a problem we'd need to look into other ways
to cache or speed up package installs.

alphagov/notifications-utils