sched_ext: Drop tools_clean target from the top-level Makefile

sched_ext: Drop tools_clean target from the top-level Makefile

2a52ca7c9896 ("sched_ext: Add scx_simple and scx_example_qmap example
schedulers") added the tools_clean target which is triggered by mrproper.
The tools_clean target triggers the sched_ext_clean target in tools/. This
unfortunately makes mrproper fail when no BTF enabled kernel image is found:

  Makefile:83: *** Cannot find a vmlinux for VMLINUX_BTF at any of "  ../../vmlinux /sys/kernel/btf/vmlinux/boot/vmlinux-4.15.0-136-generic".  Stop.
  Makefile:192: recipe for target 'sched_ext_clean' failed
  make[2]: *** [sched_ext_clean] Error 2
  Makefile:1361: recipe for target 'sched_ext' failed
  make[1]: *** [sched_ext] Error 2
  Makefile:240: recipe for target '__sub-make' failed
  make: *** [__sub-make] Error 2

Clean targets shouldn't fail like this but also it's really odd for mrproper
to single out and trigger the sched_ext_clean target when no other clean
targets under tools/ are triggered.

Fix builds by dropping the tools_clean target from the top-level Makefile.
The offending Makefile line is shared across BPF targets under tools/. Let's
revisit them later.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Jon Hunter <jonathanh@nvidia.com>
Link: http://lkml.kernel.org/r/ac065f1f-8754-4626-95db-2c9fcf02567b@nvidia.com
Fixes: 2a52ca7c9896 ("sched_ext: Add scx_simple and scx_example_qmap example schedulers")
Cc: David Vernet <void@manifault.com>