package/qoriq-rcw: drop bogus QORIQ_RCW_FILE_BIN

While applying [1] and cleaning up, the commit "d167f7006e
package/qoriq-rcw: add in-tree rcw source support" ended up keeping an
unused define.

Drop it now.

[1] http://patchwork.ozlabs.org/project/buildroot/patch/20200207083857.28058-4-jerry.huang@nxp.com/

Cc: Matt Weber <matthew.weber@collins.com>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/qoriq-rcw/qoriq-rcw.mk

@@ -59,7 +59,6 @@ endef
 else ifneq ($(QORIQ_RCW_INTREE),)
 
 QORIQ_RCW_PLATFORM = $(firstword $(subst /, ,$(QORIQ_RCW_INTREE)))
-QORIQ_RCW_FILE_BIN = $(lastword $(subst /, ,$(QORIQ_RCW_INTREE)))
 
 define HOST_QORIQ_RCW_BUILD_CMDS
 	$(MAKE) -C $(@D)/$(QORIQ_RCW_PLATFORM)