Remove legacyinputhelpers from the project's manifest.

This is a dependency of other packages, and should not be explicitly
referenced in the manifest.

Packages/manifest.json

     "com.unity.xr.arfoundation": "2.1.0-preview.2",
     "com.unity.xr.arkit": "2.1.0-preview.3",
     "com.unity.xr.arkit-face-tracking": "1.0.0-preview.3",
-    "com.unity.xr.legacyinputhelpers": "2.0.2",
     "com.unity.modules.ai": "1.0.0",
     "com.unity.modules.animation": "1.0.0",
     "com.unity.modules.assetbundle": "1.0.0",