kbuild: remove unneeded variable, single-all

When single-build is set, everything in $(MAKECMDGOALS) is a single
target. You can use $(MAKECMDGOALS) to list out the single targets.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1764,11 +1764,9 @@ tools/%: FORCE
 
 ifdef single-build
 
-single-all := $(filter $(single-targets), $(MAKECMDGOALS))
-
 # .ko is special because modpost is needed
-single-ko := $(sort $(filter %.ko, $(single-all)))
-single-no-ko := $(sort $(patsubst %.ko,%.mod, $(single-all)))
+single-ko := $(sort $(filter %.ko, $(MAKECMDGOALS)))
+single-no-ko := $(sort $(patsubst %.ko,%.mod, $(MAKECMDGOALS)))
 
 $(single-ko): single_modpost
 	@: