Remove unneeded extensions

This removes some VS Code extensions from the list in
devcontainer.json whose advantages don't clearly outweigh their
disadvantages in a dev container for this project.

That doesn't keep people from using those extensions in the dev
container if they install them, nor does it prevent them from being
installed automatically if they would be for some other reason.

Extensions I was listing before and have removed:

- markdown-preview-github-styles: Makes Markdown render the same as
  on GitHub, but we don't have a lot of Markdown, and also it will
  not always render the initial readme preview in this style, as
  that preview is shown before all extensions finish installing.

- vscode-github-actions: This extension is experimental and shows
  a lot of spurious warnings as well as indenting incorrectly (at
  least in this project).

- jupyter: Jupyter is very widely used and many peopele are likely
  to use it, even though this project doesn't ship any notebooks.
  But the extension increases load time for the dev container, and
  using Jupyter with this project already requires further changes
  because no dependency group in pyproject.toml lists ipykernel
  (nor any package that pulls it in as a dependency).

Extensions I have kept:

- gitlens: Extremely popular. Gives lots of useful Git information.

- git-graph: Visual history of commits. Especially helpful for
  projects that mostly use plain merges (rather than "rebase and
  merge" or "squash and merge").

- isort: Editor integration for isort.

- python: Self-explanatory.

- even-better-toml: Syntax highlighting for pyproject.toml.

.devcontainer/devcontainer.json

@@ -3,14 +3,11 @@
     "customizations": {
         "vscode": {
             "extensions": [
-                "bierner.markdown-preview-github-styles",
                 "eamodio.gitlens",
-                "GitHub.vscode-github-actions",
                 "GitHub.vscode-pull-request-github",
                 "mhutchie.git-graph",
                 "ms-python.isort",
                 "ms-python.python",
-                "ms-toolsai.jupyter",
                 "tamasfe.even-better-toml"
             ],
             "settings": {