Switch from recommending a local workspace extension to recommending our published extension. (#4661)

.vscode/extensions.json

@@ -2,6 +2,7 @@
   "recommendations": [
     "bazelbuild.vscode-bazel",
     "bierner.github-markdown-preview",
+    "carbon-lang.carbon-vscode",
     "esbenp.prettier-vscode",
     "llvm-vs-code-extensions.vscode-clangd",
     "ms-python.black-formatter",

.vscode/extensions/carbon-lang

@@ -1 +0,0 @@
-../../utils/vscode/