UNO_91H: remove sectors, bootloader was disabled

This should be reverted once bootloader is supported.

Author: 0xc0170

targets/targets.json

@@ -9465,7 +9465,6 @@
         "mbed_ram_size": "0x1ff80",
         "mbed_rom_start": "0x18001000",
         "mbed_rom_size": "0x1F4000",
-        "sectors": [[0,4096]],
         "overrides": {
             "network-default-interface-type" : "WIFI"
         },