Move clean/clobber to soong_ui

So that we don't have to load up all the makefile state just to remove
the output directory.

Starting from a completely empty out directory:
 kati:    16s
 soong_ui: 2.0s

From a minimal out directory (m -j blueprint_tools):
 kati:     3.8s
 soong_ui: 0.4s

Test: m -j clean
Test: m -j clobber
Change-Id: Iec1ce032c7cc7ed102430080e857fc421b66309c
diff --git a/core/config.mk b/core/config.mk
index 583909d..86915a6 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -847,7 +847,7 @@
 
 # These goals don't need to collect and include Android.mks/CleanSpec.mks
 # in the source tree.
-dont_bother_goals := clean clobber dataclean installclean \
+dont_bother_goals := dataclean installclean \
     help out \
     snod systemimage-nodeps \
     stnod systemtarball-nodeps \