Use only NUCLEO_F401RE for the NFC examples

The repository mbed-os-example-nfc contains two examples: NFC_EEPROM
and NFC_SmartPoster. According to their respective `mbed_app.json`,
only NUCLEO_F401RE supports both examples, so we limit the compilation
test to this target only.

Compilation with an unsupported target fails (as expected) with Mbed
CLI 2 which enforces a check on a target's availability of the
required NFC controller. Mbed CLI 1 lacks such check and always
compiles.

Fixes ARMmbed/mbed-os-example-nfc#106

Author: LDong-Arm

tools/test/examples/examples.json

@@ -307,7 +307,7 @@
             "NFC_SmartPoster"
         ],
         "features" : [],
-        "targets" : ["NUCLEO_F401RE", "DISCO_L475VG_IOT01A"],
+        "targets" : ["NUCLEO_F401RE"],
         "toolchains" : [],
         "exporters": [],
         "compile" : true,