cmake: removing DTS_ROOTS in test samples as they serves no purpose

This commit is a cleanup in two test samples.

In both samples, the following line of code were found:

    set(DTS_ROOTS ${CMAKE_CURRENT_LIST_DIR})

Firstly, it is not needed to set `DTS_ROOT` in samples, as the sample
folder is always added to `DTS_ROOT`.
Secondly, the variable is named `DTS_ROOT`, but the samples used a
misspelled version `DTS_ROOTS`, which means that variable has never
had any effect.

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