Nuke concat option.

[knighton]

Description of changes:

Nuke concat option in StreamingC4.

Concat results in the underlying sample positions of workers becoming disconnected, and therefore hitting epoch boundaries at different times.

Issue #, if available:

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[karan6181]

LGTM

[abhi-mosaic]

LGTM. Will pair this with a PR that improves capabilities for convert_c4.py so that we can pre-concatenate data.