[allocator.requirements.general] Remove redundant template syntax

[AlisdairM]

In the Cpp17Allocator requirements, all identifiers name specific types, even if they are template instantiations. Hence, all use of typename, .template, and ::template is redundant. Not in the sense of we are awaiting LWG to review P2150 for policy, but in the sense these have been redundant since C++11, and were outright ill-formed prior to that.

The majority of the wording throughout this subclause already (correctly) avoids use of these redundant keywords. This PR brings the title rows into agreement with the body text, and fixes one outstanding redundant use of ::template.

[JohelEGP]

I think those typename are blessed now:

(3.7)
Result: for a typename-specifier, a description of the named type; for an expression, a description of the type of the expression; the expression is an lvalue if the type is an lvalue reference type, an xvalue if the type is an rvalue reference type, and a prvalue otherwise.

[jwakely]

These aren't code, they're descriptions of members. As such, I think it's useful to show that these are types. It's how you'd write it in a requires-expression too:

requires { typename X::pointer; }

I don't think this change would be an improvement.