Unsupported targets still have dependent files specified

[Karl-G1]

Describe the bug
We removed changes to the custom device XML file from #138 when enabling the 2021 build.
This was due to the intent of making the XML file changes in #144.

Also as part of #138, we removed build support for Pharlap, Linux ARM, and VxWorks RT targets.
Because of this, the custom device shows missing dependent files in System Explorer.

Expected behavior

1. New instances of the custom device should not add those dependent files to the sysdef
2. Existing instances of the custom device should not have errors (either due to mutation or stub files)

Screenshots

Please complete the following information:
Affects the 21.0 release builld