rules: drop broken stlink util support

There was an attempt at providing support for the texane/stlink utility.
It has been broken for a long time, referring to missing scripts.
Remove it as clearly unused.

examples/rules.mk

@@ -231,7 +231,6 @@ styleclean: $(STYLECHECKFILES:=.styleclean)
 	@printf "  FLASH  $<\n"
 	$(STFLASH) write $(*).bin 0x8000000
 
-ifeq ($(STLINK_PORT),)
 ifeq ($(BMP_PORT),)
 ifeq ($(OOCD_FILE),)
 %.flash: %.elf
@@ -257,14 +256,6 @@ else
 		   -x $(EXAMPLES_SCRIPT_DIR)/black_magic_probe_flash.scr \
 		   $(*).elf
 endif
-else
-%.flash: %.elf
-	@printf "  GDB   $(*).elf (flash)\n"
-	$(GDB) --batch \
-		   -ex 'target extended-remote $(STLINK_PORT)' \
-		   -x $(EXAMPLES_SCRIPT_DIR)/stlink_flash.scr \
-		   $(*).elf
-endif
 
 .PHONY: images clean stylecheck styleclean elf bin hex srec list