support/script/scancpan: remove duplicated dependency

Dependencies from metacpan comes as a list of modules which is
transformed in a list of distribution for BR.  Different modules could
be included in the same distribution, so duplication is possible.

This can for example be seen with the HTTP-Daemon module, which would
get two times the dependencies on HTTP-Message without this commit.

[Thomas: slightly extend commit log.]

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>