Remove obsolete CUSTOM_KERNEL_HEADERS.

Change-Id: Id260dd351b90f065eee56e231d4c341b18c1b27f
diff --git a/core/combo/TARGET_linux-aarch64.mk b/core/combo/TARGET_linux-aarch64.mk
index d8a64c5..50996eb 100644
--- a/core/combo/TARGET_linux-aarch64.mk
+++ b/core/combo/TARGET_linux-aarch64.mk
@@ -133,17 +133,8 @@
 TARGET_LIBGCC := $(shell $(TARGET_CC) $(TARGET_GLOBAL_CFLAGS) \
 	-print-libgcc-file-name)
 
-# unless CUSTOM_KERNEL_HEADERS is defined, we're going to use
-# symlinks located in out/ to point to the appropriate kernel
-# headers. see 'config/kernel_headers.make' for more details
-#
-ifneq ($(CUSTOM_KERNEL_HEADERS),)
-    KERNEL_HEADERS_COMMON := $(CUSTOM_KERNEL_HEADERS)
-    KERNEL_HEADERS_ARCH   := $(CUSTOM_KERNEL_HEADERS)
-else
-    KERNEL_HEADERS_COMMON := $(libc_root)/kernel/uapi
-    KERNEL_HEADERS_ARCH   := $(libc_root)/kernel/uapi/asm-$(TARGET_ARCH)
-endif
+KERNEL_HEADERS_COMMON := $(libc_root)/kernel/uapi
+KERNEL_HEADERS_ARCH   := $(libc_root)/kernel/uapi/asm-$(TARGET_ARCH)
 KERNEL_HEADERS := $(KERNEL_HEADERS_COMMON) $(KERNEL_HEADERS_ARCH)
 
 TARGET_C_INCLUDES := \