Remove dead tag_definition method

This was last used in 753126428bbdeed87ae232165cbecbd96801dccc.

Change-Id: Ifd4b05a8bddd6addfed9b39e28c8861f7f88fc9b
Reviewed-on: http://gerrit.causes.com/42569
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>