This commit reverts a change that added support for
parsing base64-encoded keys set via `MINIO_KMS_MASTER_KEY`.
The env. variable `MINIO_KMS_MASTER_KEY` is deprecated and
should ONLY support parsing existing keys - not the new format.
Any new deployment should use `MINIO_KMS_SECRET_KEY`. The legacy
env. variable `MINIO_KMS_MASTER_KEY` will be removed at some point
in time.
Signed-off-by: Andreas Auernhammer <aead@mail.de>