remove 'tomli' dependency (#228)

Proposes removing the build-time dependency on `tomli` for wheels and conda packages.

It doesn't appear to be used anywhere here.

```shell
git grep tomli
```

## Notes for Reviewers

I originally noticed something similar in `ucx-py` (rapidsai/ucx-py#1042), then went searching for similar cases across RAPIDS.

That dependency was added for `distributed-ucxx` back in #60. I'm not sure why, but I suspect it was related to the use of `versioneer` in this project at the time. Reference: python-versioneer/python-versioneer#338 (comment)

This project doesn't use `versioneer` any more (#114). I strongly suspect that the dependency on `tomli` can be removed.

Authors:
  - James Lamb (https://github.com/jameslamb)

Approvers:
  - Lawrence Mitchell (https://github.com/wence-)
  - https://github.com/jakirkham
  - Ray Douglass (https://github.com/raydouglass)

URL: #228

conda/environments/all_cuda-118_arch-x86_64.yaml

@@ -40,6 +40,5 @@ dependencies:
 - scikit-build-core>=0.7.0
 - setuptools>=64.0.0
 - spdlog>=1.12.0,<1.13
-- tomli
 - ucx>=1.15.0
 name: all_cuda-118_arch-x86_64

conda/environments/all_cuda-122_arch-x86_64.yaml

@@ -40,6 +40,5 @@ dependencies:
 - scikit-build-core>=0.7.0
 - setuptools>=64.0.0
 - spdlog>=1.12.0,<1.13
-- tomli
 - ucx>=1.15.0
 name: all_cuda-122_arch-x86_64

conda/recipes/ucxx/meta.yaml

@@ -275,7 +275,6 @@ outputs:
         - pip
         - setuptools
         - wheel
-        - tomli
       run:
         - python * *_cpython
         - rapids-dask-dependency =24.06

dependencies.yaml

@@ -144,7 +144,6 @@ dependencies:
       - output_types: [conda, requirements, pyproject]
         packages:
           - setuptools>=64.0.0
-          - tomli
   checks:
     common:
       - output_types: [conda, requirements]

python/distributed-ucxx/pyproject.toml

@@ -2,7 +2,6 @@
 build-backend = "setuptools.build_meta"
 requires = [
     "setuptools>=64.0.0",
-    "tomli",
 ] # This list was generated by `rapids-dependency-file-generator`. To make changes, edit ../../dependencies.yaml and run `rapids-dependency-file-generator`.
 
 [project]