machine: Drop use of DEFAULT_RAM_SIZE in help text

As of commit 076b35b5a (machine: add default_ram_size to machine
class) we no longer have a global default ram size, but instead
machine specific defaults.  When invoking qemu --help we don't know
which machine you selected, so we can't tell the user the default RAM
size in the help text anymore now.

Thus I don't see an easy way to expose the default ram size to the
user in the help text.  The easiest option IMHO is to just drop this
piece of information.

Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Acked-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Message-id: 1433495103-62084-1-git-send-email-agraf@suse.de
[PMM: rewrapped long commit message lines]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
