Clean up trait bounds

[haerdib]

As @clangenb wrote in #398 (comment) :

I see us repeatedly having these excessive trait bounds. I would vote for adding them to the respective pallet's config, so we don't need to repeat them here every time.

[haerdib]

Maybe something like that: https://github.com/paritytech/subxt/blob/92ace0629a63fadeebd428c5b1356ba60eeabfeb/subxt/src/config.rs

[clangenb]

I like their approach

[haerdib]

Keep in mind: paritytech/substrate#12939. This might prove quite relevant for this issue as well. Defining "by default " parameters is not very dummy proof and will most likely cause issues in the future. With metadata V15, several of these types might be retrievable via metadata.