string: remove unused string.__append metamethod.

The __append metamethod was added in commit 7a548ba, but only
ever had one client in the defunct std.lcs module which was
removed in commet 5f0a8af.
* lib/std/string.lua (__append): Remove. No longer required.
* specs/string_spec.yaml: Remove __append examples and references.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>