MapgenBasic: Add lava source as commonly used content (#5512)

Future mapgens are likely to use this for magma and volcanos.
Remove the getting of lava source content id in mgvalleys.

src/mapgen_valleys.h

@@ -124,8 +124,6 @@ private:
 	Noise *noise_valley_depth;
 	Noise *noise_valley_profile;
 
-	content_t c_lava_source;
-
 	float terrainLevelAtPoint(s16 x, s16 z);
 
 	void calculateNoise();