exec: Remove unnecessary RAM_FILE flag

The only code that sets RAMBlock.fd is file_ram_alloc(), and the only
code that calls file_ram_alloc() sets the RAM_FILE flag. That means the
flag is always set when RAMBlock.fd >= 0, and the munmap() call at
reclaim_ramblock() is dead code that never runs.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <1446847881-9385-1-git-send-email-ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>