sdvx: remove -sdvxdisablecams, simplify camera hook (#350)

## Link to GitHub Issue, if one exists
#345

## Description of change
Simplify camera hook in SDVX5/6. Camera hook will always be installed
and return 0 cameras.

Deprecate `-sdvxdisablecams` option as it is no longer needed as a
result of this change.

## Testing
Tested SDVX5 final, SDVX6 year 1, and a recent SDVX6 build.

src/spice2x/games/sdvx/sdvx.h

@@ -17,7 +17,6 @@ namespace games::sdvx {
     };
 
     // settings
-    extern bool DISABLECAMS;
     extern bool NATIVETOUCH;
     extern uint8_t DIGITAL_KNOB_SENS;
     extern std::optional<std::string> ASIO_DRIVER;