.pre-commit-config.yaml: Do not set ANSIBLE_LIBRARY for ansible-doc-test

With latest Ansible (4.0.0) it is needed to have a complete path for
ANSIBLE_LIBRARY. It is not good to hard code this in the
.pre-commit-config.yaml file for plugins and also all roles. Instead
it will be set in ansible-doc-test as it knows the path for each file
that is checked.