Remove add subdirectory of TARGET_MCU_NRF52840 as it contains only mbed_lib.json

Author: rajkan01

connectivity/drivers/ble/FEATURE_BLE/TARGET_NORDIC/TARGET_NORDIC_CORDIO/CMakeLists.txt

@@ -1,8 +1,6 @@
 # Copyright (c) 2020 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
-if("MCU_NRF52840" IN_LIST MBED_TARGET_LABELS)
-    add_subdirectory(TARGET_MCU_NRF52840)
-elseif("NRF5x" IN_LIST MBED_TARGET_LABELS)
+if("NRF5x" IN_LIST MBED_TARGET_LABELS)
     add_subdirectory(TARGET_NRF5x)
 endif()