Remove support for pca10056-rc-clock (#4955)

In 2020, geeksville had a NRF52840-dk development board with a
busted oscilliator. Let's retire it from service :)

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

boards/nrf52840_dk_modified.json

@@ -1,47 +0,0 @@
-{
-  "build": {
-    "arduino": {
-      "ldscript": "nrf52840_s113_v7.ld"
-    },
-    "core": "nRF5",
-    "cpu": "cortex-m4",
-    "extra_flags": "-DARDUINO_NRF52840_PCA10056 -DNRF52840_XXAA",
-    "f_cpu": "64000000L",
-    "hwids": [["0x239A", "0x4404"]],
-    "usb_product": "nrf52840dk",
-    "mcu": "nrf52840",
-    "variant": "pca10056-rc-clock",
-    "variants_dir": "variants",
-    "bsp": {
-      "name": "adafruit"
-    },
-    "softdevice": {
-      "sd_flags": "-DS140",
-      "sd_name": "s140",
-      "sd_version": "6.1.1",
-      "sd_fwid": "0x00B6"
-    },
-    "bootloader": {
-      "settings_addr": "0xFF000"
-    }
-  },
-  "connectivity": ["bluetooth"],
-  "debug": {
-    "jlink_device": "nRF52840_xxAA",
-    "onboard_tools": ["jlink"],
-    "svd_path": "nrf52840.svd",
-    "openocd_target": "nrf52840-mdk-rs"
-  },
-  "frameworks": ["arduino"],
-  "name": "A modified NRF52840-DK devboard (Adafruit BSP)",
-  "upload": {
-    "maximum_ram_size": 248832,
-    "maximum_size": 815104,
-    "require_upload_port": true,
-    "speed": 115200,
-    "protocol": "jlink",
-    "protocols": ["jlink", "nrfjprog", "stlink"]
-  },
-  "url": "https://meshtastic.org/",
-  "vendor": "Nordic Semi"
-}