Remove un-used JSON attribute 'private_key_id'. Clarify format of 'private_key' parameter.

Agreed, I had to strip Bag/key attributes from the PEM file for it to be consumable by `ServiceAccountCredentials.fromJson` (which uses a PEM parsing method from the googleapis_auth package so perhaps this issue should be filed there.)

The sample openssl command addresses this, but it would be great to avoid the extra step and lower the barrier for adoption.