linux: 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>

linux/Config.in

@@ -110,7 +110,6 @@ config BR2_LINUX_KERNEL_VERSION
 
 config BR2_LINUX_KERNEL_PATCH
 	string "Custom kernel patches"
-	depends on !BR2_LINUX_KERNEL_CUSTOM_LOCAL
 	help
 	  A space-separated list of patches to apply to the
 	  kernel. Each patch can be described as an URL, a local file