[libc++] Remove ABI lists for previous releases

We don't actually update the ABI lists at every release -- it's too much
work, since we'd technically have to do it even for minor releases.
Furthermore, I don't think anybody uses those (I certainly don't rely
on them for anything).

Instead, it is better to rely on the ABI list changelog and the canonical
ABI list that we always keep up to date. If one wants to know what symbols
were shipped in a specific release, that can be discovered easily using
Git, which is a superior tool than keeping textual copies of old versions.

libcxx/NOTES.TXT

@@ -14,7 +14,6 @@ an LLVM release.
 1. Update _LIBCPP_VERSION in `__config`
 2. Update the __libcpp_version file.
 3. Update the version number in `docs/conf.py`
-4. Create ABI lists for the previous release under `lib/abi`
 
 //===---------------------------------------------------------------------===//
 // Adding a new header TODO

libcxx/lib/abi/CHANGELOG.TXT

@@ -7,8 +7,8 @@ Each entry should start with the revision number followed by a description of
 the change. The entry should contain a summary of the ABI changes made,
 including what symbols were added, removed, or changed.
 
-To generate a summary use "sym_diff.py" diffing against the appropriate ABI list.
-Afterwards the ABI list should be updated to include the new changes.
+To generate a summary, re-generate the new ABI list using the
+`generate-cxx-abilist` target, and look at the diff.
 
 New entries should be added directly below the "Version" header.