[Tiled] Fixed duplication of path in helper tilesheet loading

[kim-fg]

The provided tiled load handler duplicated path for tilesheets due to it being handled as relative to the .tmx file it is used in.

Error:

2024-03-27T21:48:21.803906Z  INFO platformer_2d::bevy_ecs_tilemap::helpers::tiled: Loaded map: tiled/dev_env/dev_env.tmx
2024-03-27T21:48:21.803980Z ERROR bevy_asset::server: Path not found: C:\<PROJECT_PATH>\assets\tiled/dev_env\tiled/dev_env\dev_env_tilesheet.png

This PR removes that code and expects img.source to be correct. This was tested both with tilesheet .png in same dir as .tmx file and in a subfolder.