Remove minimum cost of 1 doge token paid to operators and superblock submitters for doing a lock/unlock

[oscarguindzberg]

Operators nor superblock submitters have a fixed cost for letting a user cross the bridge.
So, there is no need to pass on a minimum cost to users.
Just a percentage should be keep.
If a user locks 5 doges, she should receive 4.98 doge tokens.