fix: remove explicit param from GossipSubParams constructor

[diegomrsantos]

I believe explicit is used to check if the GossipSubParams instance was created by the user either passing params to GossipSubParams(...) or GossipSubParams.init(...). In the first case explicit should be set to true when calling the Nim constructor, in the second case, the param isn't necessary and should be always be set to true by init. If none of those options were used, it means the instance was created by using Nim default values. In this case, GossipSubParams.init() should be called to set the values to their default value defined by nim-libp2p.
https://github.com/status-im/nim-libp2p/blob/5e55ca3b69e18a56a0a5a34941676f4df08523ae/libp2p/protocols/pubsub/gossipsub.nim#L774

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.80%. Comparing base (03f67d3) to head (403449d).

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1080      +/-   ##
============================================
- Coverage     84.82%   84.80%   -0.02%     
============================================
  Files            91       91              
  Lines         15428    15428              
============================================
- Hits          13087    13084       -3     
- Misses         2341     2344       +3     

Files	Coverage Δ
libp2p/protocols/pubsub/gossipsub.nim	86.46% <ø> (ø)

... and 1 file with indirect coverage changes