Ditch unused `.dictionaries tbody` color rule

This style is shadowed by the more specific `.full tbody` rule in tables.less:

https://github.com/mozilla/olympia/blob/181dc72ca40352f63ba01fe76f29836e4ca0dba2/static/css/impala/tables.less#L20-22

Might as well nuke the unused one.
Merge pull request #297 from callahad/master

Ditch unused `.dictionaries tbody` color rule