Remove work-around in dpctl.SyclPlatform.get_devices for custom and automatic device types

[ndgrigorian]

This PR removes a work-around added to dpctl.SyclPlatform.get_devices due to a bug in the implementation, where the device types "automatic" and "custom" would return "all"

To work-around, these were treated as "unknown" and returned an empty list instead of a full list

The work-around is no longer necessary

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[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞

[coveralls]

coverage: 75.476%. remained the same — remove-platform-get-devices-work-around into master

[vlad-perevezentsev]

LGTM
Thank you @ndgrigorian