www/seamonkey: remove port

Upstream has poor history of delivering security fixes on time.
2.49.4 was released almost 1 year ago. While 2.49.5 with 60.2
backports is planned[1] it's at least 1 month away while ESR60
will reach EOL in 2 months. By the time 2.57.0 arrives it'll
also be vulnerable.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1512882

https://hg.mozilla.org/releases/mozilla-esr52/shortlog/SEAMONKEY_2_49_5_RELEASE