[libcxx] [llvm] Remove workarounds for NDK versions before 2017's r16. (PR #148879)

[via llvm-commits]

enh-google wrote:

> If Android is already taking the `__linux__` path, then IMO this change makes sense as-is.

yeah, the _other_ copy of this file (which i was going to clean up separately, so let me know if you'd prefer to have all the changes in one PR) doesn't have a `__linux__` case: https://github.com/llvm/llvm-project/blob/8612926c306c5191a5fb385dd11467728c59e982/libcxx/include/__cxx03/__locale_dir/locale_base_api.h

so this code is actually still used in _that_ copy (and in older versions of this copy), but it looks like your recent cleanup (https://github.com/llvm/llvm-project/commit/363bfd6090b0755fc133250ba3470b420902d976) accidentally made this dead code --- but that was the right thing to do anyway, so no-one noticed :-)

(i'm only here because brad0 has been cleaning up various "we don't need that any more" Android hacks around the tree, and this was the only place i found that was testing for specific versions of the NDK, not because this caused any trouble.)

https://github.com/llvm/llvm-project/pull/148879