samples: dfu: use settings to store credentials by default

[mniestroj]

Make some cleanup across sample code that is used for more than a single sample. See commits for details.

So far Golioth credentials were baked into the firmware after configuring those with Kconfig (when used with DFU). Start using settings subsystem to store them instead, so that single firmware can be used for multiple devices.

Disable settings subsystem for qemu_x86 platform, as there is no persisent storage configured with default configuration used.

[github-actions[bot]]

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