[module.interface] Redundant wording after P2109R0

[zygoloid]

After the application of http://wiki.edg.com/pub/Wg21prague/StrawPolls/p2109r0.html we have:

A declaration is exported if it is:

• a namespace-scope declaration declared within an export-declaration, or
• a module-import-declaration within an export-declaration, or

... but the case covered by the second bullet is now also covered by the first bullet. Let's remove the second bullet.