tests: posix: headers: remove tests for varying features

The header tests for the existence of constants like
_POSIX_SPIN_LOCKS are bound to fail in one circumstance or
another because each of those options are (theoretically)
features which can be enabled or disabled.

Moreover, some implementations (e.g. external libc) might
not implement one particular subset or option group.

These tests are poorly conditioned, so remove them.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>