mail/mailslurper: update to 1.14.2.g20201217

Upgrade mailslurper to the latest github commit and remove patches
that have been merged upstream. The last commit is from 2020, but no
new release has appeared.

PR: 268270
Reported by: einar@isnic.is (maintainer)
Approved by: eduardo (mentor)
Differential Revision: https://reviews.freebsd.org/D38520