layers: Remove an unused & unimplemented function

`layer_getKey` was introduced in 6d641e7fc5, by
mistake, it was never implemented. Remove it now to avoid any possible
confusion.

Signed-off-by: Gergely Nagy <kaleidoscope@gergo.csillger.hu>

src/layers.h

@@ -75,6 +75,4 @@ class Layer_ {
   static uint8_t activeLayers[ROWS][COLS];
 };
 
-Key layer_getKey(uint8_t layer, uint8_t r, uint8_t c);
-
 extern Layer_ Layer;