BUG/MINOR: check: do not reset check flags on purge

BUG/MINOR: check: do not reset check flags on purge

Do not reset check flags when setting CHK_ST_PURGE.

Currently, this change has no impact. However, it is semantically wrong
to clear important flags such as CHK_ST_AGENT on purge.

Furthermore, this change will become mandatoy for a future fix to
properly free agent checks on dynamic servers removal. For this, it will
be needed to differentiate health/agent-check on purge via CHK_ST_AGENT
to properly free agent checks.

This must not be backported unless dynamic servers checks are
backported.