www/chromium shouldn't be marked as forbidden in Makefile

[Yuri]

There is /usr/ports/security/vuxml/vuln.xml listing this vulnerability, 
and build already by default fails due to this unless user sets special 
variable to waive this. So there is no need to also forbid it through 
the Makefile.

www/firefox and thunderbird are also currently prevented from building 
due to security vulnerabilities.

Yuri