Revert "(#12844) Backwards compatibility for 'puppet agent --enable'"

This reverts commit fcac8f7163c99884fc6b75e3851c4a5e16a3ff07, which
was a backwards-compatibility workaround intended to handle some new
behavior related to puppet agent lockfiles that was introduced in
2.7.10. The fix is being reverted because we've decided to remove
the new lockfile behavior from the 2.7.x series entirely, and push
it out to 3.x.