mail/imapfilter: Drop LibreSSL patch and unbreak build with OpenSSL 1.1.1

The patch added in ports r435498 broke OpenSSL 1.1.x compatibility.
A different fix for LibreSSL was committed upstream so the patch
is no longer needed since imapfilter 2.6.11 [1].

[1] https://github.com/lefcha/imapfilter/commit/c1865ba1ab2fd8da14e5e018ab55c79cbc432295

PR: 232132
Approved by: bofh (maintainer timeout, 1 month)