perf: simplify join func (#4434)

Signed-off-by: Yue Yang <g1enyy0ung@gmail.com>

cmd/generate-makefile/doc.go

@@ -13,5 +13,6 @@
 // limitations under the License.
 //
 
-// This command line tool would generate Makefile like binary.generated.mk, contains targets for building executable binaries.
+// This command line tool is used to generate partial Makefiles, such as binary.generated.mk,
+// containing targets for building executable binaries.
 package main

cmd/generate-makefile/funcmaps.go

@@ -16,8 +16,4 @@ package main
 
 import "strings"
 
-var defaultFuncMap = map[string]interface{}{"StringsJoin": StringsJoin}
-
-func StringsJoin(s []string, sep string) string {
-	return strings.Join(s, sep)
-}
+var defaultFuncMap = map[string]interface{}{"StringsJoin": strings.Join}