samples: tests: remove obsolete KERNEL_TYPE and kernel variables

Remove those from Makefiles and testcase.ini, we now support unified kernel
only and sanitycheck script now knows how to deal with this.

Change-Id: I853ebcadfa7b56a4de5737d95f2ba096babb2e13
Signed-off-by: Anas Nashif <anas.nashif@intel.com>