Clean up references to the `carbon-toolchain` repository. (#226)

Now that we've centralized on a monorepo, tidy up some lingering
references.

For the `pr_comments.py` script, I've left the flag in place to select
a repository as it seems useful functionality to have available even if
we don't expect to need it in the near term. That said, I can pull it
out if folks prefer.

I haven't updated any of the *proposals* because it seems better to
leave those as-is from when they were written. The references seem
unlikely to be confusing to me. Happy for suggestions if needed there
though.

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

README.md

@@ -136,11 +136,3 @@ compatibility and interoperability with C++:
 the language as well as large-scale migration for users.** This is extremely
 expensive, and so the bar for Carbon to be a compelling direction for C++ is
 very high.
-
-## Repository structure overview
-
-Carbon's main repositories are:
-
--   **carbon-lang** - Carbon language specification and documentation.
--   **carbon-toolchain** - Carbon language toolchain and reference
-    implementation.

docs/design/code_and_name_organization/README.md

@@ -839,8 +839,7 @@ Advantages:
 -   Allow greater flexibility and hierarchy for related packages, such as
     `Database.Client` and `Database.Server`.
 -   Would allow using GitHub repository names as package names. For example,
-    `carbon-language/carbon-toolchain` could become
-    `carbon_language.carbon_toolchain`.
+    `carbon-language/carbon-lang` could become `carbon_language.carbon_lang`.
 
 Disadvantages:
 

github_tools/pr_comments.py

@@ -285,7 +285,7 @@ def _parse_args(args=None):
     )
     parser.add_argument(
         "--repo",
-        choices=["carbon-lang", "carbon-toolchain"],
+        choices=["carbon-lang"],
         default="carbon-lang",
         help="The Carbon repo to query. Defaults to %(default)s.",
     )