Remove audit.meta.category

[paulirish]

As discussed here #3227 (comment) .. it's not needed and we can drop it.

[arturmiz]

I can take a look on this one and do the cleanup :)

[Siilwyn]

@arturmiz hi I'm using the audit category in my own parser, maybe I should switch to only using what's under the reportCategories key...

But for now, how should I get the associated category given an audit now? Any ideas appreciated. =)

[patrickhulce]

@Siilwyn yes you should use the category it's associated with in reportCategories, the audit.meta.category was not the real "category" you see in the report and was a somewhat irrelevant and old display string :) sorry for the churn!

[Siilwyn]

@patrickhulce thank you for your reply! I already found it odd that it used different names, so I wrote a mapping function to convert them to the correct category. :)