Disable fee levels when sending strat

[mikedennis]

Since there is only one fee level for strat we could disable these options except for bitcoin to avoid confusion. I've seen this reported a couple of different times in the community testing with the beta.

[dev0tion]

Yeah I agree, people get confused because it's the same fee for all types of transactions.
What do you think @bokobza?

[bep42]

We just have to display it without the selection option :)