db: remove the 6.x recipe

db: remove the 6.x recipe

Version 6.x of Berkeley DB has been rejected by open source community due to its hostile
AGPLv3 license; both Fedora and Debian are sticking with db 5.x - and by extension,
all the open source projects are still developed and tested with db 5.x

In oe-core the only thing that was requiring db 6.x was rpm 5.x, and so there's no reason
to continue carrying db 6.x in oe-core. If someone needs API features that are only available in
db 6.x, it can be re-added to meta-oe.

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>