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.

Start with src/main/java/com/amazonaws/encryptionsdk/internal/FrameDecryptionHandler.java at the linked decrypt-path code, then search for the partial variable-frame-length handling and related references. Remove the unsupported feature and verify that the remaining decryption behavior is covered by the repository’s existing tests; the issue does not name a specific test file.

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