Fix API user registration role

[bobbyiliev]

Removing the role_id causing the bug as reported in #184

The default role is assigned already thanks to:

wave/app/Models/User.php

Lines 62 to 69 in a96ac49

	// Listen for the created event of the model
	static::created(function ($user) {
	// Remove all roles
	$user->syncRoles([]);
	// Assign the default role
	$user->assignRole( config('wave.default_user_role', 'registered') );
	});
	}