libatomic: drop redundant all-multi command

[Jeff Law]

On 5/27/2022 2:01 AM, Jan Beulich via Gcc-patches wrote:
> ./multilib.am already specifies this same command, and make warns about
> the earlier one being ignored when seeing the later one. All that needs
> retaining to still satisfy the preceding comment is the extra
> dependency.
>
> libatomic/
> 2022-05-XX  Jan Beulich  <jbeulich@suse.com>
>
> 	* Makefile.am (all-multi): Drop commands.
> 	* Makefile.in: Update accordingly.
OK for the trunk.
jeff