Removing existing dirs from .gitignore

There are a couple of dirs present in the neutron tree that have
erroneously been added to the .gitignore file with [0]. Drop those so
that content there may be regularly updated again.

Affected directories:

- bin/
- neutron/tests/functional/agent/linux/bin/
- devstack/lib/
- neutron/tests/var/

[0] https://review.opendev.org/632094

Change-Id: Id56f61965bfc4ba3dcce3319cfda467d89f897b1

.gitignore

@@ -22,12 +22,9 @@ dist
 build
 eggs
 parts
-bin
-var
 sdist
 develop-eggs
 .installed.cfg
-lib
 lib64
 
 # Installer logs