boot/ti-k3-r5-loader: drop bogus TI_K3_R5_LOADER_BOARD

Commit "0189bcb47c boot/ti-k3-r5-loader: new package" added an unused
define.
The define value is calculated from an inexistent option
BR2_TARGET_TI_K3_R5_LOADER_BOARD, so drop the line.

Cc: Anand Gadiyar <gadiyar@ti.com>
Cc: Xuanhao Shi <X15000177@gmail.com>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/ti-k3-r5-loader/ti-k3-r5-loader.mk

@@ -26,7 +26,6 @@ TI_K3_R5_LOADER_KCONFIG_DEPENDENCIES = \
 	$(BR2_BISON_HOST_DEPENDENCY) \
 	$(BR2_FLEX_HOST_DEPENDENCY)
 
-TI_K3_R5_LOADER_BOARD = $(call qstrip,$(BR2_TARGET_TI_K3_R5_LOADER_BOARD))
 ifeq ($(BR2_TARGET_TI_K3_R5_LOADER_USE_DEFCONFIG),y)
 TI_K3_R5_LOADER_KCONFIG_DEFCONFIG = $(call qstrip,$(BR2_TARGET_TI_K3_R5_LOADER_BOARD_DEFCONFIG))_defconfig
 else ifeq ($(BR2_TARGET_TI_K3_R5_LOADER_USE_CUSTOM_CONFIG),y)