This reverts commit 9c01d875f3.

This is very confusing and messy.. but it's documented.
`ansible_become_*` variables actually take priority and override all `become_*`
variables set at the task level.

As such, using `ansible_become=true ansible_become_user=root` in
`inventory/hosts` causes issues because tasks that specify
`become: OTHER_USER` will be forced to run as `root` due to
`ansible_become_user`.