Cleanup: Strict compiler warning about capturing constexpr value

Resolves the following error when compiling with Apple Clang:

```
lambda capture 'thickness_adjustement_factor' is not required to be captured for this use [-Wunused-lambda-capture]
```

Pull Request: https://projects.blender.org/blender/blender/pulls/121012

source/blender/blenkernel/intern/grease_pencil_convert_legacy.cc

@@ -1227,7 +1227,7 @@ static void layer_adjustments_to_modifiers(ConversionData &conversion_data,
 
   /* Ensure values are divided by 2k, to match conversion done for non-animated value. */
   constexpr float thickness_adjustement_factor = 1.0f / 2000.0f;
-  auto fcurve_convert_thickness_cb = [&thickness_adjustement_factor](FCurve &fcurve) {
+  auto fcurve_convert_thickness_cb = [&](FCurve &fcurve) {
     if (fcurve.bezt) {
       for (uint i = 0; i < fcurve.totvert; i++) {
         BezTriple &bezier_triple = fcurve.bezt[i];