build: Don't delete llvm files as part of make clean

Doing this is just resulting in a broken workspace. We have a clean-llvm
target that does the right thing. If we want to we can make it a dependency
of clean, but most people don't need to clean llvm and doing so results
in a lengthy rebuild.

mk/clean.mk

@@ -33,10 +33,10 @@ clean-misc:
 	$(Q)rm -Rf $(GENERATED)
 	$(Q)rm -f rustllvm/$(CFG_RUSTLLVM) rustllvm/rustllvmbits.a
 	$(Q)rm -f rt/$(CFG_RUNTIME)
-	$(Q)find rustllvm llvm rt -name '*.[odasS]' -delete
-	$(Q)find rustllvm llvm rt -name '*.so' -delete
-	$(Q)find rustllvm llvm rt -name '*.dylib' -delete
-	$(Q)find rustllvm llvm rt -name '*.dll' -delete
+	$(Q)find rustllvm rt -name '*.[odasS]' -delete
+	$(Q)find rustllvm rt -name '*.so' -delete
+	$(Q)find rustllvm rt -name '*.dylib' -delete
+	$(Q)find rustllvm rt -name '*.dll' -delete
 	$(Q)find rustllvm rt -name '*.def' -delete
 	$(Q)rm -Rf $(wildcard rt/*/libuv/Default)
 	$(Q)rm -f test/run_pass_stage2.rc test/run_pass_stage2_driver.rs