docs: remove old unused favicon file

The use of 32favicon.png was removed when the new favicons were
introduced in

  commit 40cb5581
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Wed Jul 26 18:22:11 2017 +0100

    docs: add full set of "favicon" files to support modern clients
Reviewed-by: NLaine Stump <laine@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

docs/Makefile.am

@@ -151,7 +151,6 @@ logofilesdir = $(HTML_DIR)/logos
 logofiles_DATA = $(logofiles)
 
 assets = \
-  32favicon.png \
   android-chrome-192x192.png \
   android-chrome-256x256.png \
   apple-touch-icon.png \