Remove partially implemented variable frame length feature

A feature to allow variable frame lengths for framed data was partially implemented in the decrypt path long ago. Variable frame lengths would be indicated by the frame length in the header being 0 (see https://github.com/aws/aws-encryption-sdk-java/blob/7d5ab6eef428364286c50b969ef89766be6207ee/src/main/java/com/amazonaws/encryptionsdk/internal/FrameDecryptionHandler.java#L122).

This feature is not documented anywhere or supported in other ESDKs, so it should be removed.