bug: remove buggy auto-generated CPU colour implementation

[ClementTsang]

Description

A description of the change and what it does. If relevant, please provide screenshots of what results from the change:

Removes the random automatically generated colours for the CPU metrics. This was not supported in all terminal emulators, and would cause some of them to break (namely macOS Terminal).

Instead we'll default to colours we can be more certain will work and loop through them as required. Users can still override these colours with their own.

Issue

If applicable, what issue does this address?

Closes: #307

Type of change

Remove the irrelevant ones:

• Bug fix (non-breaking change which fixes an issue)

Test methodology

If relevant, please state how this was tested:

Furthermore, please tick which platforms this change was tested on:

• Windows
• macOS
• Linux

If relevant, all of these platforms should be tested.

Checklist

If relevant, ensure the following have been met:

• Change has been tested to work, and does not cause new breakage unless intended
• Code has been self-reviewed
• Documentation has been added/updated if needed (README, help menu, etc.)
• Passes Travis tests (clippy check and cargo test check)
• Areas your change affects have been linted using rustfmt (cargo fmt)
• No merge conflicts arise from the change

Other information

Provide any other relevant information to this change:

[ClementTsang]

Tested by SSHing into my 16-core Linux machine from my MBP. Didn't seem to be broken.