Update to support LMIC v3.1.0

Looks like the upstream adruino library [released v3.1.0][v310] two weeks ago.

From [the changeset][changes], this includes Helium patches that add the pinmap
for `Disco_L072cz_Lrwan1`. Unfortunately, that means that when this code compiles,
it now throws duplicate symbol errors. This simply removes the now redundant
definition and fixes the type where needed to match upstream.

[v310]: https://github.com/mcci-catena/arduino-lmic/releases/tag/v3.1.0
[changes]: 3b9b02d..79accbb

longfi-us915/longfi-us915.ino

@@ -106,10 +106,7 @@ const lmic_pinmap lmic_pins = {
 #include "arduino_lmic_hal_boards.h"
 const lmic_pinmap lmic_pins = *Arduino_LMIC::GetPinmap_Catena4610();
 #elif defined(ARDUINO_DISCO_L072CZ_LRWAN1)
-namespace Arduino_LMIC {
-const HalPinmap_t GetPinmap_Disco_L072cz_Lrwan1();
-}
-const lmic_pinmap lmic_pins = Arduino_LMIC::GetPinmap_Disco_L072cz_Lrwan1();
+const lmic_pinmap lmic_pins = *Arduino_LMIC::GetPinmap_Disco_L072cz_Lrwan1();
 #else
 #error "Unknown target"
 #endif
@@ -321,6 +318,4 @@ static const HalPinmap_t myPinmap = {
     .spi_freq = 8000000, /* 8MHz */
     .pConfig = &myConfig};
 
-const HalPinmap_t GetPinmap_Disco_L072cz_Lrwan1(void) { return myPinmap; }
-
 }; // end namespace Arduino_LMIC