Remove hypenated formatting from set of accepted digest algorithms

> Given the accepted community standards, I don't believe we should accept hyphenated cases
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> SRC: https://github.com/iipc/warc-specifications/blob/master/specifications/warc-format/warc-1.1-annotated/index.md#warc-block-digest

_Originally posted by @willmhowes in [e7fd7b4](https://github.com/internetarchive/gowarc/commit/e7fd7b4c5e956a2a803dda96063af3c6f8e45f0d#r164667158)_

Start by reviewing the digest-algorithm acceptance logic and the cited WARC 1.1 specification; the payload does not name a source file or test. Check linked pull request #132 for prior work, and consider the issue done when hyphenated algorithm forms are no longer accepted in accordance with the referenced standard.

Written by the indexing model from the issue text.