Remove fgets() compatbility function and related tests.

This patch removes the `tor_fgets()` wrapper around `fgets(3)` since it
is no longer needed. The function was created due to inconsistency
between the returned values of `fgets(3)` on different versions of Unix
when using `fgets(3)` on non-blocking file descriptors, but with the
recent changes in bug #21654 we switch from unbuffered to direct I/O on
non-blocking file descriptors in our utility module.

We continue to use `fgets(3)` directly in the geoip and dirserv module
since this usage is considered safe.

This patch also removes the test-case that was created to detect
differences in the implementation of `fgets(3)` as well as the changes
file since these changes was not included in any releases yet.

See: https://bugs.torproject.org/21654