Retire devel/go-shellwords

Details: port had been introduced as a stop-gap solution to satisfy
a build dependency to textproc/fzf. In the meantime, textproc/fzf has
been reworked to satisfy the dependency. No other consumers for
devel/go-shellwords are present in the ports tree at this point in time.

PR: 214698
Submitted by: sascha@root-login.org (maintainer of devel/go-shellwords)