Remove sdflib plugin and move interpolation to engine_collision_sdf.

Before:
```
 Simulation time      : 1.71 s
 Steps per second     : 5854
 Realtime factor      : 11.71 x
 Time per step        : 170.8 µs

 Newton iters / step  : 2.19
 Contacts / step      : 3.45
 Constraints / step   : 13.79
 Degrees of freedom   : 12
 Dynamic memory usage : 0.2% of 14M
```

After:
```
 Simulation time      : 1.41 s
 Steps per second     : 7093
 Realtime factor      : 14.19 x
 Time per step        : 141.0 µs

 Newton iters / step  : 2.19
 Contacts / step      : 3.45
 Constraints / step   : 13.79
 Degrees of freedom   : 12
 Dynamic memory usage : 0.2% of 14M
```

PiperOrigin-RevId: 781087896
Change-Id: Iaf69dccc5e95af5bac862344683c74f962162186

plugin/sdf/README.md

@@ -62,16 +62,6 @@ Parameters:
   - `radius1` [m]: major radius (default `0.35`).
   - `radius1` [m]: minor radius (default `0.15`).
 
-### SdfLib
-
-Implemented in [sdflib.cc](sdflib.cc). Example usage in [cow.xml](../../model/plugin/sdf/cow.xml).
-
-This plugin uses the library [TriangleMeshDistance](https://github.com/InteractiveComputerGraphics/TriangleMeshDistance)
-to compute a voxel-based approximation of a user-specified mesh. The mesh can be arbitrary and not necessarily convex.
-This offers an alternative to convex-decomposed meshes. The performance is likely to be slower than that of analytic
-SDFs, since a cubic approximation has to be evaluated on the convex grid. However, the SDF generation is done
-automatically, simplifying the task of creating an SDF, which can be difficult for complex shapes.
-
 ### How to make your own SDF
 
 Create your `MySDF.h` and `MySDF.cc` files in the SDF folder, where this README is located. Implement your SDF using the