Ditch unused `.dictionaries tbody` color rule

`(body).dictionaries tbody { color: #555; }` is shadowed by the more specific `(section).full tbody { color: @dark-gray; }` in [tables.less](https://github.com/mozilla/olympia/blob/181dc72ca40352f63ba01fe76f29836e4ca0dba2/static/css/impala/tables.less#L20-22):

![Web inspector screenshot](https://cloud.githubusercontent.com/assets/24193/4410522/9efc634a-44e6-11e4-8f97-4b9e3bd1ca59.png)

Might as well nuke the unused rule in dictionaries.less.