mail/fetchmail: update to 6.4.24 and block LibreSSL.

Now really 6.4.24 and not a 6.4.25 WIP.

fetchmail cannot legally be linked with LibreSSL,
because there is no GPLv2 clause 2b exemption for
LibreSSL, only for OpenSSL.

Correct LICENSE and remove LICENSE_COMB.
Add comment on FSF dynamic linking dynamically
suggested by Corey Halpin in the approval.

Remove LibreSSL patch.

Related to:
PR: 259214

Update:
PR: 259945
MFH: 2021Q4

Approved by: chalpin@cs.wisc.edu (maintainer)