Remove default address prefix value of AminoTypes

[webmaster128]

In

/**
 * A map from Stargate message types as used in the messages's `Any` type
 * to Amino types.
 */
export class AminoTypes {
  private readonly register: Record<string, AminoConverter>;

  public constructor({ additions = {}, prefix = "cosmos" }: AminoTypesOptions = {}) {

there is a default prefix set. But whenever you need to construct this explicitly, you are most likely not the Cosmos Hub. Using "cosmos" as a default makes it easy to forget setting the field correctly.