linux-headers: cleanup dead custom-local option

The option `BR2_LINUX_KERNEL_CUSTOM_LOCAL` no longer exists (see commit
e782cd5b1bc231dda527d5d0a04e6a338669b92c [1]); removing the option. Note
that this legacy option has already been handled (Config.in.legacy) in
the mentioned commit.

Signed-off-by: James Knight <james.knight@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/linux-headers/linux-headers.mk

@@ -17,9 +17,6 @@ LINUX_HEADERS_TARBALL = $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_TARBALL_LOCATION
 LINUX_HEADERS_SITE = $(patsubst %/,%,$(dir $(LINUX_HEADERS_TARBALL)))
 LINUX_HEADERS_SOURCE = $(notdir $(LINUX_HEADERS_TARBALL))
 BR_NO_CHECK_HASH_FOR += $(LINUX_HEADERS_SOURCE)
-else ifeq ($(BR2_LINUX_KERNEL_CUSTOM_LOCAL),y)
-LINUX_HEADERS_SITE = $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_LOCAL_PATH))
-LINUX_HEADERS_SITE_METHOD = local
 else ifeq ($(BR2_LINUX_KERNEL_CUSTOM_GIT),y)
 LINUX_HEADERS_SITE = $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_REPO_URL))
 LINUX_HEADERS_SITE_METHOD = git