Drop Pirate Audio default pin config for #9

This config is set by the Pirate Audio install script- https://github.com/pimoroni/pirate-audio/blob/master/mopidy/install.sh

Since this library is device agnostic, these defaults don't have to be here, and shouldn't be here. Users should have to explicitly configure and enable the plugin.

See #9 for an example of the default pin config being unexpectedly loaded and causing the plugin to bail.

mopidy_raspberry_gpio/ext.conf

@@ -5,8 +5,8 @@ bcm1 =
 bcm2 = 
 bcm3 = 
 bcm4 = 
-bcm5 = play_pause,active_low,250
-bcm6 = volume_down,active_low,250
+bcm5 = 
+bcm6 = 
 bcm7 = 
 bcm8 = 
 bcm9 = 
@@ -16,11 +16,11 @@ bcm12 =
 bcm13 = 
 bcm14 = 
 bcm15 = 
-bcm16 = next,active_low,250
+bcm16 = 
 bcm17 = 
 bcm18 = 
 bcm19 = 
-bcm20 = volume_up,active_low,250
+bcm20 = 
 bcm21 = 
 bcm22 = 
 bcm23 =