don't depend on /plugins/ folder for installation of pip packages

remove check for plugins directory

macros.install_pip doesn't need the check
as it's handling empty package lists already

the check was introduced in Id75c387668c6277efae05b59063f226843aa2a25
when there was no macro, but an unconditional direct pip install
call.

to spell it out for posterity, currently macro.install_pip does:

{%- if packages is sequence and packages|length > 0 -%}
    python{{ python_version }} -m pip --no-cache-dir install --upgrade{{ ' ' }}

while before pip was called directly:

pip --no-cache-dir install --upgrade -c requirements/upper-constraints.txt /plugins/*;

see: https://review.opendev.org/c/openstack/kolla/+/401145/3/docker/keystone/keystone/Dockerfile.j2#11