Unset stale SSH_TTY environment variable

[ProducerMatt]

This variable would confuse devices that relied on it, such as the Clipetty extension for Emacs. See issue #1134.

Tested on my end, I can now use Clipetty without needing to detect Mosh's existence and unset the variable manually. LMK if I did anything incorrectly.

[carloscabanero]

This environment variable may also interfere with other commands that check if they are being run within a SSH session (ie, https://github.com/theimpostor/osc). Because the SSH_TTY dies with the SSH command, it does not make sense for mosh to inherit it.

I tried to remove this on the client side when calling mosh-server, but the result was not portable.

The patch is consistent with the previous "STY" unset. Is there anything I can do to help merge this?

[bionade24]

+1 for merging this. Please give some explanation if you don't want to merge this and close the PR.

[ProducerMatt]

@Chutdafugkup sorry for accidentally requesting a review. I can't tell what your review was (an approval, a comment, etc) and I hit the request button while fumbling around trying to figure it out.