Remove dead use-postage-snapshot flag and associated code

[martinconic]

The --use-postage-snapshot flag in bee appears to be non-functional and dead code.

This flag is intended to bootstrap the node by fetching a postage snapshot from a Swarm feed. However, the snapshotFeed address in
pkg/node/bootstrap.go
is hardcoded to a zero address (0000...).

// pkg/node/bootstrap.go
var (
    // zeroed out while waiting to be replacement for the new snapshot feed address
    snapshotFeed = swarm.MustParseHexAddress("0000000000000000000000000000000000000000000000000000000000000000")
)

Because the address is zeroed out, the node cannot fetch a snapshot from this feed, rendering the feature essentially broken and unused.