`-fexperimental-assignment-tracking` was added in fdd8b03d4b to
make coroutine debugging work.
However, since then, it became unnecessary, perhaps due to 87c0adb2fe,
or perhaps to a toolchain fix.
Drop it, so we can benefit from assignment tracking (whatever it is),
and to improve compatibility with sccache, which rejects this option.
I verified that the test added in fdd8b03d4b fails without the option
and passes with this patch; in other words we're not introducing a
regression here.
Closesscylladb/scylladb#27763
b105ad8379