[NFC][libc++][test] Removes C++98 support. (#92930)

Libc++ has no separate C++98 support, it uses C++03 instead. This
removes some obsolete c++98 markers in the test.

Thanks to @StephanTLavavej for spotting this.

NOKEYCHECK=True
GitOrigin-RevId: f98a3dd7a23ca1a85f62b38a2a7e82a53f80070b
15 files changed