Jenkinsfile.kernel : remove the kernel build scripts

They are now located here

https://source.puri.sm/Librem5/kernel-builder
2 jobs for drop_builders in 18 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #10684
librem5
job

00:00:18

failed #10526
librem5
job

00:00:04