qemu: block: Remove unused flags QEMU_BLOCK_STORAGE_SOURCE_BACKEND_PROPS_ flags

qemu: block: Remove unused flags QEMU_BLOCK_STORAGE_SOURCE_BACKEND_PROPS_ flags

QEMU_BLOCK_STORAGE_SOURCE_BACKEND_PROPS_SKIP_UNMAP is no longer
referenced inside the code.

QEMU_BLOCK_STORAGE_SOURCE_BACKEND_PROPS_AUTO_READONLY is passed from
various code paths to the qemuBlockStorageSourceGetBackendProps helper,
but it's no longer used.

Both thus can be removed.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>