Remove redundant FIXME comment

[organic1337]

Background

While searching for potential points in the code to which I could contribute I came across this FIXME comment
which seemed like an easy-win:

        // FIXME: linear time search, do we care?

(line 511 in html5ever/src/tokenizer/mod.rs)

Fix attempt

I Tries using a HashSet for the attributes names so the search's complexity would be O(1) instead of O(n), as
can be seen in this commit

and once I ran some benchmarks on multiple websites (by modifying the noop-tokenizer example) I realized I only made things worst, the Parsing process took a tiny bit longer than before. This is probably because of the overhead within maintaining a HashSet of the attributes names, and because most elements do not have enough attributes for the HashSet to improve performance.

Conclusion

I think it's a good Idea to remove this comment so no one else will try to optimize this section for no reason.

[jdm]

Thanks for the analysis!