Feature: cib,libcrmcommon: drop support for PCMK_legacy variable

It was only used to select "CIB legacy mode" (v1 patchset format) on hosts that
otherwise would use the current patchset format. We aren't dropping support for
legacy mode itself, which will still be used in rolling upgrades from 1.1.11,
but this will ensure that future rolling upgrades from >=2.0.0 clusters will
never need legacy mode.