kbuild: remove duplicated comments about PHONY

The comment is the same as in the top-level Makefile.

Also, the comments contain typos:
  - the .PHONY variable  ->  the PHONY variable
  - se we can ...        ->  so we can ...

Instead of fixing the typos, just remove the duplicated comments.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile.clean

@@ -88,7 +88,4 @@ PHONY += $(subdir-ymn)
 $(subdir-ymn):
 	$(Q)$(MAKE) $(clean)=$@
 
-# Declare the contents of the .PHONY variable as phony.  We keep that
-# information in a variable se we can use it in if_changed and friends.
-
 .PHONY: $(PHONY)