Remove unused doc directives related to categories.

[srawlins]

The categories feature has a number of doc directives that have only ever been half supported: {@api}, {@samples}, and {@image}. Dartdoc only goes so far as to relay data from these directives into a categories.json file (see generateCategoryJson), but they aren't used in the web side. (Maybe we just needed to make them available for custom templates?)

Anyhow I think we can now drop support for all three. The analyzer now parses directives and reports when it sees an "unknown" directive. It does not have support for these three directives, and we have not seen any bugs that it reports their use.