Name clean tasks consistently

Makefile

@@ -31,7 +31,7 @@ methods := \
 	random_forest \
 	softmax_regression \
 	sparse_coding
-clean_methods := $(methods:%=clean-%)
+clean_methods := $(methods:%=clean_%)
 
 # Must come first so that it's the default target
 .PHONY: all
@@ -51,7 +51,7 @@ clean_helper:
 
 .PHONY: $(clean_methods)
 $(clean_methods):
-	$(MAKE) -C src/methods/$(@:clean-%=%) clean
+	$(MAKE) -C src/methods/$(@:clean_%=%) clean
 
 .PHONY: clean
 clean: clean_helper $(clean_methods)