[libc++][test] Small fixes for time tests (#132532)
* Fix copy-paste damage in `formatter.tai_time.pass.cpp`.
+ Comparing the `#if` to the `#else` case, it's clear that this
half-copied check should be removed.
* Mark `is_steady` as `[[maybe_unused]]`.
+ It's only used within `LIBCPP_STATIC_ASSERT`.
