libssh: Remove progressmeter

It is used only by scp and sftp, and already included directly in their
Makefiles. It does not belong in libssh.

Fixes: d8b043c8d497 ("Update for 3.6.1p1; also remove Kerberos IV shims.")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48871