Remove mention of UMD support for web workers

[nickbabcock]

OMT only supports amd or esm as output, so the readme should not mention umd as supported.

When executing microbundle with umd format and --workers, the web worker will fail to be detected or bundled.

[changeset-bot[bot]]

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Latest commit: e83f3b82ed26c12a425c991d3923cb3dc5902fc3

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