doc: getting_started: Remove redundant and erronous doc's

The 'getting started' documentation is stating that one should set
some environment variables, but this is not necessary because the user
has already been instructed to set the variables in the
platform-specific guides.

The duplicated documentation should be removed because it is inferiour
to the original documentation. E.g. this documentation does not
describe how to permanently set environment variables. Also, it is
confusingly demonstrating how to use the SDK on Windows, but this is
not supported.

I believe that the purpose of the section is to verify that the user
has not misconfigured or misinstalled the toolchain, but this
responsibility is handled better by CMake itself.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>