package: cleanup github <pkg>{_VERSION, _SOURCE, _SITE} variable

According to the documentation, chapter
6.2.8.2 How to add a package from github
"
[...]
FOO_VERSION = tag or full commit ID
FOO_SITE = http://github.com/<user

>/<package>/tarball/$(FOO_VERSION)
[...]
- The tarball name generated by github matches the default one from
  Buildroot (...),
  so it is not necessary to specify it in the +.mk+ file.
"
This commit makes the appropriate changes.

Signed-off-by: Jerzy Grzegorek <jerzy.grzegorek@trzebnica.net>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>