sched_ext: Drop tools_clean target from the top-level Makefile

2a52ca7c ("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: 2a52ca7c ("sched_ext: Add scx_simple and scx_example_qmap example schedulers")
Cc: David Vernet <void@manifault.com>

Makefile

@@ -1355,12 +1355,6 @@ ifneq ($(wildcard $(resolve_btfids_O)),)
 	$(Q)$(MAKE) -sC $(srctree)/tools/bpf/resolve_btfids O=$(resolve_btfids_O) clean
 endif
 
-tools-clean-targets := sched_ext
-PHONY += $(tools-clean-targets)
-$(tools-clean-targets):
-	$(Q)$(MAKE) -sC tools $@_clean
-tools_clean: $(tools-clean-targets)
-
 # Clear a bunch of variables before executing the submake
 ifeq ($(quiet),silent_)
 tools_silent=s
@@ -1533,7 +1527,7 @@ PHONY += $(mrproper-dirs) mrproper
 $(mrproper-dirs):
 	$(Q)$(MAKE) $(clean)=$(patsubst _mrproper_%,%,$@)
 
-mrproper: clean $(mrproper-dirs) tools_clean
+mrproper: clean $(mrproper-dirs)
 	$(call cmd,rmfiles)
 	@find . $(RCS_FIND_IGNORE) \
 		\( -name '*.rmeta' \) \