[Keyboard] Fix compilation issues for Ploopy Trackball classic (#15364)

keyboards/ploopyco/trackball/trackball.h

@@ -33,8 +33,6 @@
 #define OPT_ENC1_MUX 0
 #define OPT_ENC2_MUX 4
 
-void process_wheel(report_mouse_t* mouse_report);
-
 #define LAYOUT(BL, BM, BR, BF, BB) \
     { {BL, BM, BR, BF, BB}, }