wayland: Don't add the nanosecond timestamp offset to the pre-conversion millisecond value

(cherry picked from commit 6c61a94a4b)

src/video/wayland/SDL_waylandevents.c

@@ -199,7 +199,6 @@ static Uint64 Wayland_EventTimestampMSToNS(Uint32 wl_timestamp_ms)
         timestamp_offset += SDL_MS_TO_NS(SDL_UINT64_C(0x100000000));
     }
     last = wl_timestamp_ms;
-    wl_timestamp_ms += timestamp_offset;
 
     return SDL_MS_TO_NS(wl_timestamp_ms) + timestamp_offset;
 }