UNO_91: disable bootloader support as device name missing

The config failes with an exception that was previously ignored.

Author: 0xc0170

targets/targets.json

@@ -9460,7 +9460,6 @@
         "inherits": ["RDA5981X"],
         "detect_code": ["8001"],
         "components_add": ["FLASHIAP"],
-        "bootloader_supported": true,
         "mbed_ram_start": "0x00100080",
         "mbed_ram_size": "0x1ff80",
         "mbed_rom_start": "0x18001000",