build: Depend on the published forge2d 0.15.1

The length unit support in flame-engine/forge2d#120 was released as
0.15.1, so the git dependency override is no longer needed.

Author: spydon

packages/flame_forge2d/README.md

@@ -50,7 +50,7 @@ Some more documentation can be found
 
 ## Migrating from 0.19
 
-Version 0.20 is built on Forge2D 0.16, which replaced the pure Dart port of Box2D 2.x with
+Version 0.20 is built on Forge2D 0.15, which replaced the pure Dart port of Box2D 2.x with
 bindings for Box2D v3, so the whole API changed. See the [flame_forge2d migration
 guide](https://docs.flame-engine.org/main/bridge_packages/flame_forge2d/migration.html) and the
 [Forge2D migration

packages/flame_forge2d/pubspec.yaml

@@ -20,7 +20,7 @@ dependencies:
   flame: ^1.38.0
   flutter:
     sdk: flutter
-  forge2d: ^0.16.0
+  forge2d: ^0.15.1
 
 dev_dependencies:
   dartdoc: ^9.0.0

pubspec.yaml

@@ -125,14 +125,3 @@ melos:
       packageFilters:
         scope: flame_devtools
       description: Runs the devtools in the simulated mode.
-
-# TODO(spydon): Remove once forge2d 0.16.0 is published.
-# flame_forge2d needs `initializeForge2D(lengthUnitsPerMeter:)` and
-# `Tolerances`, which are in flame-engine/forge2d#120 and not in the published
-# 0.15.0.
-dependency_overrides:
-  forge2d:
-    git:
-      url: https://github.com/flame-engine/forge2d.git
-      ref: feat/length-units-per-meter
-      path: packages/forge2d