Gridmap + BakedLightmap produces huge scene files

Godot version: 3.2.2

OS/device including version: Windows 10 x64

Issue description: When you use a BakedLightmap in conjunction with a Gridmap, a huge amount of data is added to the scene in the form of baked_meshes

Such as:

[node name="GroundGridmap" parent="." index="10"]
baked_meshes = [ SubResource( 536 ), SubResource( 537 ), SubResource( 538 ), SubResource( 539 ), SubResource( 540 ), SubResource( 541 ), SubResource( 542 ), SubResource( 543 ), SubResource( 544 ), SubResource( 545 ), SubResource( 546 ), SubResource( 547 ), SubResource( 548 ), SubResource( 549 ), SubResource( 550 ), SubResource( 551 ), SubResource( 552 ), SubResource( 553 ) ]

[sub_resource type="ArrayMesh" id=561]
lightmap_size_hint = Vector2( 328, 328 )
surfaces/0 = {
"aabb": AABB( 62.0751, -0.018673, -268.127, 1.99954, 2.09549, 1.7879 ),
"array_data": PoolByteArray( ... )
"blend_shape_data": [  ],
"format": 97591,
"index_count": 696,
"material": ExtResource( 17 ),
"primitive": 4,
"skeleton_aabb": [  ],
"vertex_count": 242
}

As TSCN this can add hundreds of MB to the file. Even when saved as a binary SCN these files are still dozens of MB for larger gridmaps.

Additionally, if the BakedLightmap node is removed from the scene, these baked_mesh resources don't get removed, so the scene file will never go back down in size.

Steps to reproduce: Add a Gridmap node to a scene, place tons of tiles down. Note that the scene is still quite small. Add a BakedLightmap node to the scene that covers the whole gridmap, and bake the light map. Now note that the scene file size has ballooned to many MB.

Finally delete the BakedLightmap and note that the size does not go back down again.

Minimal reproduction project: Having trouble setting this up as the assets I have to work with keep crashing the 3.2.2 light mapper.

godotengine / godot

Gridmap + BakedLightmap produces huge scene files #41009