kbuild: drop unifdef-y support

unifdef-y is not used anymore - drop remaining references
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Documentation/kbuild/makefiles.txt

@@ -45,7 +45,6 @@ This document describes the Linux kernel Makefiles.
 		--- 7.1 header-y
 		--- 7.2 objhdr-y
 		--- 7.3 destination-y
-		--- 7.4 unifdef-y (deprecated)
 
 	=== 8 Kbuild Variables
 	=== 9 Makefile language
@@ -1245,11 +1244,6 @@ See subsequent chapter for the syntax of the Kbuild file.
 	will be located in the directory "include/linux" when exported.
 
 
-	--- 7.4 unifdef-y (deprecated)
-
-	unifdef-y is deprecated. A direct replacement is header-y.
-
-
 === 8 Kbuild Variables
 
 The top Makefile exports the following variables:

scripts/Makefile.headersinst

@@ -3,7 +3,6 @@
 #
 # header-y  - list files to be installed. They are preprocessed
 #             to remove __KERNEL__ section of the file
-# unifdef-y - Same as header-y. Obsolete
 # objhdr-y  - Same as header-y but for generated files
 #
 # ==========================================================================
@@ -20,7 +19,7 @@ include scripts/Kbuild.include
 
 install       := $(INSTALL_HDR_PATH)/$(_dst)
 
-header-y      := $(sort $(header-y) $(unifdef-y))
+header-y      := $(sort $(header-y))
 subdirs       := $(patsubst %/,%,$(filter %/, $(header-y)))
 header-y      := $(filter-out %/, $(header-y))