Fix the build with gcc when `-Wredundant-decls` is passed

Summary:

gcc warns that `__throw_runtime_error` is declared both in
`<__locale>` and `<stdexcept>`, if `-Wredundant-decls` is passed on
the command line; this is the case with FreeBSD when ${WARNS} == 6.

Since `<__locale>` gets its first declaration via a transitive
include of `<stdexcept>`, and the second declaration is after the
first invocation of `__throw_runtime_error`, delete that second
declaration.

Signed-off-by: Enji Cooper <yaneurabeya@gmail.com>

Reviewers: kristina, MaskRay, EricWF, ldionne, ngie

Reviewed By: EricWF

Subscribers: krytarowski, brooks, emaste, dim, christof, jdoerfert, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D58425