[libc++][NFC] Remove workaround for variadic templates in C++03

That's not necessary anymore, since we always build the dylib with
C++11 capabilities nowadays.

Differential Revision: https://reviews.llvm.org/D107772

NOKEYCHECK=True
GitOrigin-RevId: 706bd129b35c012860fd381482cfbc066e02639a
1 file changed