uboot-tools: drop unreferenced empty variable

Commit log 0e0ea8cf5e2c57 (uboot-tools: install libubootenv to staging)
mentions that installation is done in <pkg>_INSTALL_STAGING_CMDS directly, but
forgot to remove the now empty UBOOT_TOOLS_INSTALL_LIBUBOOTENV.

Cc: Jörg Krause <joerg.krause@embedded.rocks>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/uboot-tools/uboot-tools.mk

@@ -68,9 +68,6 @@ define UBOOT_TOOLS_INSTALL_DUMPIMAGE
 endef
 endif
 
-define UBOOT_TOOLS_INSTALL_LIBUBOOTENV
-endef
-
 define UBOOT_TOOLS_INSTALL_STAGING_CMDS
 	$(INSTALL) -D -m 0755 $(@D)/tools/env/lib.a $(STAGING_DIR)/usr/lib/libubootenv.a
 	$(INSTALL) -D -m 0644 $(@D)/tools/env/fw_env.h $(STAGING_DIR)/usr/include/fw_env.h