Bug fix: Only try to find decoder for zip, 7z once

[Mark Martinec via amavis-users]

Quanah,

> Currently at startup, Amavis tries to find a decoder for 7z and zip
> extensions twice:
> -    [[qw(7z zip gz bz2 Z tar)],
> +    [[qw(gz bz2 Z tar)],

Thank you, will get that into the final 2.9.0.

   Mark