fedora: Drop SPDX identifier from spec file

...which makes it fall under MIT licensing terms. Daniel reports that
it's very unusual for spec files to contain explicit licensing terms
and might cause minor inconveniences later on, on mass changes to
spec files.

I originally added licensing information using SPDX identifiers to
make the project fully compliant with the REUSE Specification 3.0
(https://reuse.software/spec/), but there are anyway a few more files
not including explicit licensing information. It might be worth to
fix that later on, in any case.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

contrib/fedora/passt.spec

@@ -1,5 +1,3 @@
-# SPDX-License-Identifier: AGPL-3.0-or-later
-#
 # PASST - Plug A Simple Socket Transport
 #  for qemu/UNIX domain socket mode
 #