fix(assets): remove unreferenced duplicate planet_surface texture

assets/textures/planet_surface.png was byte-identical to and unused
in favor of assets/models/nebula_planet_planet_surface.png, the
sidecar Godot's glTF importer actually extracted from nebula_planet.glb
and uses at runtime. Deleted the orphan (+ its .import) and stopped
gen_planet_surface.py from writing it.

Also measured nebula_dust.gdshader's per-fragment depth-texture sample
cost (~0.07ms/frame at 500 particles, within noise) -- negligible, so
no budgeting concern for further particle work.

Game/assets/textures/planet_surface.png.import

@@ -1,40 +0,0 @@
-[remap]
-
-importer="texture"
-type="CompressedTexture2D"
-uid="uid://bxsadfd04v6r2"
-path="res://.godot/imported/planet_surface.png-d3aaf8310508e4c85b26a3d093bcf9c8.ctex"
-metadata={
-"vram_texture": false
-}
-
-[deps]
-
-source_file="res://assets/textures/planet_surface.png"
-dest_files=["res://.godot/imported/planet_surface.png-d3aaf8310508e4c85b26a3d093bcf9c8.ctex"]
-
-[params]
-
-compress/mode=0
-compress/high_quality=false
-compress/lossy_quality=0.7
-compress/uastc_level=0
-compress/rdo_quality_loss=0.0
-compress/hdr_compression=1
-compress/normal_map=0
-compress/channel_pack=0
-mipmaps/generate=false
-mipmaps/limit=-1
-roughness/mode=0
-roughness/src_normal=""
-process/channel_remap/red=0
-process/channel_remap/green=1
-process/channel_remap/blue=2
-process/channel_remap/alpha=3
-process/fix_alpha_border=true
-process/premult_alpha=false
-process/normal_map_invert_y=false
-process/hdr_as_srgb=false
-process/hdr_clamp_exposure=false
-process/size_limit=0
-detect_3d/compress_to=1