[associative] Remove redundant template arguments.

This removes redundant template arguments from [map.overview],
[multimap.overview], [set.overview] and [multiset.overview].

This is consistent with other member functions that rely on the injected
class name and I find it much easier to read the class synopses without
the clutter of the explicit template argument lists.
Merge pull request #42 from jwakely/associative

[associative] Remove redundant template arguments. Thanks to Jonathan Wakely!