Removed background of most png's to use them later as an entity

...a/de/hdm_stuttgart/battlearena/Model/Map/TileManager.java

src/main/resources/textures/map/bigbomb.png

src/main/resources/textures/map/bigbomba.png

src/main/resources/textures/map/bigexplosion.png

src/main/resources/textures/map/bomba.png

src/main/resources/textures/map/finalheart.png

src/main/resources/textures/map/heart.png

src/main/resources/textures/map/middleexplosion.png

src/main/resources/textures/map/middlexplosion.png

src/main/resources/textures/map/smallexplosion.png