Necesarry as default when make is called without dkms.
If called with dkms, luckily the variable and value passed as command
line parameter overides the one defined in the makefile as desired. So
removing was not necesarry, details in:
https://www.gnu.org/software/make/manual/make.html#Using-Variables
The KDIR variable assignment gets overwritten by the KDIR=... in the
make invocation started from dkms. So it gets never used.
(See:
https://stackoverflow.com/questions/2826029/passing-additional-variables-from-command-line-to-make
)
Even if it would be used it would lead to compilation errors, since KDIR
has to be defined by dkms (when it "cross-compiles" for other kernels), not by
uname with our current kernel
