nrf: clean up code after CGo improvements

[aykevl]

Make use of a few new features in TinyGo:

• Functions and globals in header files are supported.
• Static functions are supported.

This allows us to remove workarounds specifically used for the lack of support for static functions.

This is a draft because it depends on https://github.com/tinygo-org/tinygo/pull/2776.