QTPy_M0 variant has LED_BUILTIN but there is none

[jerryneedell]

The variant defines the LEB_BUILTIN on Pin 13 , but there is no LED on the board.
https://github.com/adafruit/ArduinoCore-samd/blob/master/variants/qtpy_m0/variant.h#L82

This is not really a problem except that things like the "blink" sketch compile but do't actually work.

[hathach]

since LEB_BUILTIN is widely used in most examples, I guess it is a way to prevent compile error. Not sure how to "fix" this though.

[ladyada]

i think we could remove it