remove U_HAVE_STD_ATOMICS and similar @internal macros

Description

ICU 64+ no longer uses the U_HAVE_STD_ATOMICS macro, but we still have

and similar internal macros.

Remove obsolete macros marked as internal.

Fixed
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Assignee

Andy Heninger

Reporter

Markus Scherer

Components

Priority

medium

Time Needed

Minutes

Fix versions