Remove the use of commit

[nashqueue]

message Commit {
	repeated bytes signatures = 1;
}

message SignedHeader {
	Header header = 1;
	Commit commit = 2;

We currently use Commit in Signed Header even though it only has one field. (rollkit.proto) We should remove it and have Signed Header be Header + Signature.

In the same Pr, we can remove the commit in types/block.go .