Fix incorrect style serialization when using raster DEM tile sources

[karimnaaji]

Fix for #10416, introducing incorrect diffing of styles when using raster dem tile sources, which resulted in reloading the scene in Studio when making any sort of style changes while using terrain.

Before vs After in studio:

bug.mov

fix.mov

Launch Checklist

• briefly describe the changes in this PR
• [n/a] include before/after visuals or gifs if this PR includes visual changes
• write tests for all new functionality
• [n/a] document any changes to public APIs
• [n/a] post benchmark scores
• manually test the debug page
• [n/a] tagged @mapbox/map-design-team @mapbox/static-apis if this PR includes style spec API or visual changes
• [n/a] tagged @mapbox/gl-native if this PR includes shader changes or needs a native port
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Fix incorrect diffing of styles when using raster DEM tile sources</changelog>

[arindam1993]

LGTM and nice fix, actually removing code!