boards/um_feathers3: Don't flash tinyuf2.bin (#807)

tinyuf2.bin should only be flashed along with the appropriate bootloader
and partition scheme. In arduino IDE, these are only flashed when the
tinyuf2 partition option is selected.

Author: prplz

boards/um_feathers3.json

@@ -35,14 +35,6 @@
   ],
   "name": "Unexpected Maker FeatherS3",
   "upload": {
-    "arduino": {
-      "flash_extra_images": [
-        [
-          "0x410000",
-          "variants/um_feathers3/tinyuf2.bin"
-        ]
-      ]
-    },
     "flash_size": "16MB",
     "maximum_ram_size": 327680,
     "maximum_size": 16777216,