xtensa: Remove 64 bits atomic builtins stubs

The stubs were provided by commit 725aea87 ("xtensa: enable KCSAN")
to make linker happy allthought they are not meant to be used at all.

KCSAN core has been fixed to not require them anymore on
32 bits architectures.

Then they can be removed.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/a6834980e58c5e2cdf25b3db061f34975de46437.1683892665.git.christophe.leroy@csgroup.eu