Stop setting USE_PYTHON3

This was removed from DevStack in the 2024.2 cycle [1], but was unused since
way back in Victoria [2].

[1] openstack/devstack@5412dbf
[2] openstack/devstack@d7a82f4

Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

Author: stephenfin

action.yaml

@@ -61,7 +61,6 @@ runs:
         SERVICE_PASSWORD=secret
         SWIFT_HASH=1234123412341234
         LOGFILE=${{ inputs.log_dir }}/devstack.log
-        USE_PYTHON3=True
         INSTALL_TEMPEST=False
         GIT_BASE=https://github.com
         EOF