build: Remove obsolete comments referring to Autotools

Autotools support were removed by commit
9837f974 ("Drop autotools").

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Christophe de Dinechin <dinechin@redhat.com>
2 jobs for master in 3 minutes and 32 seconds
Status Job ID Name Coverage
  Test
passed #460822
fedora

00:03:31

passed #460823
windows

00:02:00