Remove legacy kernel build toolchain PATH setup in envsetup.sh

We used to rely on envsetup.sh to set up PATH before doing kernel build.
Now we have new kernel build process.
Also the variable doesn't work any more as we don't have corresponding arm-eabi
toolchain for the latest arm-linux-androideabi toolchain.

Change-Id: I99443fdc58e94f57f4f1f7e88da6dd127f3b2fac