libsystemd/sd-id128: use only internal hmac, remove khash/OpenSSL support

Using OpenSSL brings in an additional dependency for all users of
libsystemd.so even though it's just one API that makes use of it.

The khash implementation is awkward as it requires context switches and
computation inside the kernel, thus leaving the process.

Remove both from libsystemd.so, and use exclusively the internal hmac fallback.
While this is not optimized, the sd-id128 API is not used in
performance-critical contexts where hardware acceleration would make a
noticeable difference.

LICENSES/README.md

@@ -58,3 +58,8 @@ The following exceptions apply:
    **BSD-3-Clause** license.
  * any files under test/ without an explicit license we assume non-copyrightable
    (eg: computer-generated fuzzer data)
+
+## OpenSSL Notes
+
+Note that building the systemd project with OpenSSL does not affect the libsystemd.so
+shared library, which is not linked with the OpenSSL library.