tests: delete libhostname.so and chkhostname

Before this patch, `libhostname.so` and `chkhostname` were a test
facility for overriding `gethostname()` in non-debug builds on
Linux and other Unix platforms supporting `LD_PRELOAD`.

`gethostname()` has a single use with SMTP.

The alternative way to override `gethostname()` is building in debug
mode, which allows to do this via the `CURL_GETHOSTNAME` env, on all
platforms.

Drop the `LD_PRELOAD` solution in favour of the above.

Also:
- delete inactive NTLM code with a `gethostname()` call made from it.
- streamline NTLM code by dropping a `printf()` and a macro.
- tests: stop setting `CURL_GETHOSTNAME` where unnecessary.

Closes #14695

configure.ac

@@ -4064,14 +4064,6 @@ AM_CONDITIONAL(USE_MANUAL, test x"$USE_MANUAL" = x1)
 
 CURL_CHECK_LIB_ARES
 
-if test "x$curl_cv_native_windows" != "xyes" &&
-   test "x$enable_shared" = "xyes"; then
-  build_libhostname=yes
-else
-  build_libhostname=no
-fi
-AM_CONDITIONAL(BUILD_LIBHOSTNAME, test x$build_libhostname = xyes)
-
 if test "x$want_ares" != xyes; then
   CURL_CHECK_OPTION_THREADED_RESOLVER