boards: samples: remove CONFIG_BUILD_NO_GAP_FILL=y and similar

Gap filling in hex files are now disabled per default, and therefore
there is no reason to explicitly disable gap filling.

It has never been possible to disable gap filling in binary files.
Disabling gap filling would just result in the binary file to be gap
filled with the tool's default value, objcopy=0x00.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>