Attribute interpolation: remove info about old syntax

[VojtaStanek]

When quickly scanning the docs, some (including me) can spot the old syntax in the docs (a(href="/#{url}") Link) which was removed in 2.0 and without reading the text around it (which says it was removed), they can go and use it.

I propose removing this information from this page of the docs, maybe with link "There used to be some other syntax, see migration guide for more information".
I think that the information about usage of template strings and other ways of attr interpolation should not be removed.

[kylekatarnls]

Pugjs 2 is not even yet stable (RC4 right now) and you already want to remove info that will help a lot of people to migrate? I can find 4-years deprecated function like mysql_connect in the PHP documentation, it's not why I will use it. We should not drop pug 1 users support just to help some users too huried to read the feature doc before use it.

A good documentation also talk about breaking changes in new versions.

[ForbesLindesay]

I think this issue is caused by a breaking change in twitter bootstrap, which we currently use for our website. The note about the old syntax is in a div with the class alert alert-danger which should create a big angry red box around it. Once we fix that, it should be pretty obvious that it's a note about old, outdated syntax.