posix: system_database_r: remove invalid compilation filters

pwd.c and grp.c both had incorrect preprocessor guards around the stubs
that they were implementing. Functions were surrounded by

```cpp
ifdef CONFIG_POSIX_THREAD_SAFE_FUNCTIONS
..
endif /* CONFIG_POSIX_THREAD_SAFE_FUNCTIONS */
```

Which is not at all accurate, since that subprofiling option group is
`(CONFIG_)POSIX_SYSTEM_DATABASE_R` (which is a PR that is in-draft).

Remove the guards, since they are invalid anyway, and at most an
application will link properly. We don't really need the guards anyway,
since conditional compilation will be used to link the files into the
build.

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