[lex.ext] Remove redundant requirement

[lprv]

This requirement is redundant with the more general provision found in [basic.lookup.general].

[AlisdairM]

I believe there are bigger structural problems here. Paragraphs 2--7 are all phase 7 material about how to lookup and dispatch a call to a literal operator, that is not needed for [lex] that does not use the actual value of the literal. I suggest that a correct fix would move all of those paragraphs to a more appropriate location.