Remove libiconv duplicate

Commit ee2c3ab ("Remove spurious libiconv dependencies") pruned all
existing "depends_on 'libiconv'" usages from Homebrew in preparation for
removing the libiconv dupe itself.

Now that is done, and we can remove and blacklist it. It can be obtained
from Homebrew-alt.

Closes Homebrew/homebrew#10464.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>