qemu-doc: Remove paragraph about requiring a HD image with -kernel

The need for specifying "-hda" together with "-kernel" has been removed in
commit 57a46d057995 ("Convert linux bootrom to external rom and fw_cfg"),
almost 10 years ago, so let's remove this description from our documentation
now, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20191001110111.4870-1-thuth@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

qemu-doc.texi

@@ -906,10 +906,6 @@ Use @option{-kernel} to provide the Linux kernel image and
 @option{-append} to give the kernel command line arguments. The
 @option{-initrd} option can be used to provide an INITRD image.
 
-When using the direct Linux boot, a disk image for the first hard disk
-@file{hda} is required because its boot sector is used to launch the
-Linux kernel.
-
 If you do not need graphical output, you can disable it and redirect
 the virtual serial port and the QEMU monitor to the console with the
 @option{-nographic} option. The typical command line is: