graphics/gnome-color-manager: build and install manual pages (again)

Now that `textproc/docbook-utils' port had been undeprecated, revert
part of the previous commit that disabled manpage generation. While
here, adjust LICENSE to match the SPDX-License-Identifier (GPL-2.0+)
and garbage-collect dependencies for calibration tools which had not
been maintained, did not work well, and thus were removed five years
ago (upstream commit 5a1bf35e by Richard Hughes).

Fixes: 093b402a7dba, e7431c909036