Revert "p/postfix_mail: fix incorrect delivered message count"

This reverts commit d07e7c15d11cebbcdf03c3171f406cd01f39e460.

Reopens: D:675318
p/postfix_mailstats: proper fix, but partial

It only partially handles D:675318, as postfix_mailvolume is not fixed.
That plugin needs a special handling, since the information is on 2 different
lines, and it's not an one-line fix.
Merge branch 'bts/675318-incorrect-postfix-delivery-count' into stable-2.0
ChangeLog: 2.0.24