bug: subscribe to default pubsub topic even when not configured

[jm-clius]

Problem

Nwaku will always subscribe to the default pubsub topic /waku/2/default-waku/proto even when explicitly configured not to.

Impact

This prevents nwaku nodes unable/unwilling to participate on the default pubsub topic (with high bandwidth requirements) from subscribing only to a different pubsub topic of their choice. This is a current requirement for developers: waku-org/pm#13

To reproduce

./build/wakunode2 --topics:/waku/2/dev-waku/proto

Note logs. Node subscribes to both /waku/2/dev-waku/proto and (incorrectly) the default waku/2/default-waku/proto.

Expected behavior

./build/wakunode2 (no arguments): subscribe to default pubsub topic waku/2/default-waku/proto.
./build/wakunode2 --topics:/waku/2/dev-waku/proto: subscribe only to the configured topic /waku/2/dev-waku/proto
./build/wakunode2 --topics:"/waku/2/default-waku/proto /waku/2/dev-waku/proto": subscribe to both configured topics, namely waku/2/default-waku/proto and /waku/2/dev-waku/proto.

Note this config item itself can be revisited to perhaps be a repeated item rather than space separated list, but this falls beyond the scope of fixing this bug.

nwaku version/commit hash

nwaku v0.14.0 and earlier.