Chain INPUT (policy DROP 0 packets, 0 bytes)
This line here shows that the default policy (i.e. what happens if none of the rules below are matched) on traffic coming into the server is dropped.
If you clear (flush) all the rules in the INPUT policy, all traffic coming into the server will be dropped (since the default is to drop packets that don't match any rules).
Change the default for INPUT to accept, and place a rule at the end of the INPUT chain to drop all traffic (in effect, to drop all traffic that hasn't yet matched a rule/exception above it), rather than set the default policy to drop.