Remove target.json DEVICE_DMESG_BUFFER_SIZE value to use the defaults.

[microbit-carlos]

So that the value can be automatically be set to zero if DEVICE_DMESG is not defined or set 1.

This PR goes together with:

• Set default DEVICE_DMESG_BUFFER_SIZE based on DEVICE_DMESG. codal-core#160

And is discussed in:

• The only way for DMESG to be a nop operation is setting DEVICE_DMESG_BUFFER_SIZE to 0 #297

[github-actions]

Build diff

Base commit: be35cc1d13697a783e08658a94e3af2b6c1afc4e
Action run: https://github.com/lancaster-university/codal-microbit-v2/actions/runs/4620890718

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[microbit-carlos]

Reviewed on a call with @finneyj and @JohnVidler and ready to be merged.