WeightInfo for Session Pallet

[shawntabrizi]

polkadot companion: paritytech/polkadot#1735

This is a PR that adds WeightInfo to the Session Pallet as generated by our automated pipeline.

Note that before, the formula for weight was a function of:

T::Keys::key_ids().len()

However, this length is static for the runtime. In the case of the Substrate node, it is:

impl_opaque_keys! {
	pub struct SessionKeys {
		pub grandpa: Grandpa,
		pub babe: Babe,
		pub im_online: ImOnline,
		pub authority_discovery: AuthorityDiscovery,
	}
}

So the length is 4. When running benchmarks, we will use this static length and calculate the reads and writes, and these are consistent with the results from the automated pipeline.

For every runtime, if there is a different set of keys or it changes, they need to rebenchmark this pallet.

This PR also removes WeightInfo from the Offences pallet since it is not used or needed. Pallets that implement OnOffenceHandler will return the appropriate weight to the Offences pallet.

[gui1117]

looks good to me

[apopiak]

LGTM

[shawntabrizi]

bot merge

[ghost]

Trying merge.