deb: remove duplicated database sections

It seems that "Package"s uses "Section: database" in "Source" section
as the default "Section".

I counldn't find the behavior in the following Debian Policy Manual
sections:

  * http://www.debian.org/doc/debian-policy/ch-archive.html#s-subsections
  * http://www.debian.org/doc/debian-policy/ch-controlfields.html#s-f-Source

But it will be true because an official Debian developer's diff says
it.

[groonga-dev,01930]

Patch by Hideki Yamane. Thanks!!!