Couple `snap_manageAccounts` to `endowment:keyring`

snap_manageAccounts is only supposed to be used by Snaps that implement the keyring API, thus we don't need two separate permissions for this.

We should figure out a way to make snap_manageAccounts not a permission anymore, but instead an RPC method gated by endowment:keyring. E.g.

snaps/packages/snaps-rpc-methods/src/permitted/experimentalProviderRequest.ts

Lines 155 to 157 in a4f3df3

    
           if (!hasPermission(SnapEndowments.EthereumProvider)) { 
        
             return end(rpcErrors.methodNotFound()); 
        
           }

This needs to be done in a backwards compatible way.

	if (!hasPermission(SnapEndowments.EthereumProvider)) {
	return end(rpcErrors.methodNotFound());
	}