remove guitar cruft

There was a bunch of code for handling guitar-specific input such as hammer-ons/pull-offs and strumming. Given that guitar was removed, and as far as I understand, will not be added back into SM5, it makes sense to remove code that had no other use.

src/ScreenGameplay.cpp

@@ -2574,13 +2574,6 @@ bool ScreenGameplay::Input( const InputEventPlus &input )
 					if( iCol != -1 )
 						pi.m_pPlayer->Step( iCol, -1, input.DeviceI.ts, false, bRelease );
 					return true;
-				case GameButtonType_Fret:
-					if( iCol != -1 )
-						pi.m_pPlayer->Fret( iCol, -1, input.DeviceI.ts, false, bRelease );
-					return true;
-				case GameButtonType_Strum:
-					pi.m_pPlayer->Strum( iCol, -1, input.DeviceI.ts, false, bRelease );
-					return true;
 				}
 			}
 		}