[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.
e.g. it's much easier to see that 

    map& operator=(map&& x);

is a move assignment operator rather than

    map<Key,T,Compare,Allocator>&
      operator=(map<Key,T,Compare,Allocator>&& x);