Reland "Move some prebuilt build tool configs to Soong"

Now that they're defined with prebuilt_build_tool, we don't need to set
them here.

In future changes we can replace more of these definitions with
prebuilt_build_tool, as it can centralize the selection of
build-from-source or prebuilt for Make, Soong, and user-defined
genrules.

Test: treehugger
Change-Id: I5821bbad1b655d561919245320d7c184a6eac737
diff --git a/core/main.mk b/core/main.mk
index 4578d90..37b2367 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -33,6 +33,8 @@
 
 include $(SOONG_MAKEVARS_MK)
 
+YACC :=$= $(BISON) -d
+
 include $(BUILD_SYSTEM)/clang/config.mk
 
 # Write the build number to a file so it can be read back in