kernel: Remove support for kernel 3.18

No target is using kernel 3.18 anymore, remove all the generic
support for kernel 3.18.

The removed packages are depending on kernel 3.18 only and are not used on
any recent kernel.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

package/kernel/linux/modules/video.mk

@@ -187,7 +187,7 @@ define KernelPackage/fb-tft
   SUBMENU:=$(VIDEO_MENU)
   TITLE:=Support for small TFT LCD display modules
   DEPENDS:= \
-	  @GPIO_SUPPORT @!LINUX_3_18 @!LINUX_4_9 +kmod-backlight \
+	  @GPIO_SUPPORT @!LINUX_4_9 +kmod-backlight \
 	  +kmod-fb +kmod-fb-sys-fops +kmod-fb-sys-ram +kmod-spi-bitbang
   KCONFIG:= \
        CONFIG_FB_BACKLIGHT=y \
@@ -433,7 +433,7 @@ endef
 
 define KernelPackage/video-videobuf2
   TITLE:=videobuf2 lib
-  DEPENDS:=+kmod-dma-buf @!LINUX_3_18
+  DEPENDS:=+kmod-dma-buf
   KCONFIG:= \
 	CONFIG_VIDEOBUF2_CORE \
 	CONFIG_VIDEOBUF2_MEMOPS \