Bison/Flex are now installed and built by bazel.

docs/project/contribution_tools.md

@@ -23,7 +23,6 @@ contributions.
         -   [Cargo (optional)](#cargo-optional)
 -   [Main tools](#main-tools)
     -   [Bazel and Bazelisk](#bazel-and-bazelisk)
-    -   [Bison and Flex](#bison-and-flex)
     -   [buildifier](#buildifier)
     -   [Clang and LLVM](#clang-and-llvm)
     -   [Ninja](#ninja)
@@ -156,24 +155,6 @@ Our recommended way of installing is:
 brew install bazelisk
 ```
 
-### Bison and Flex
-
-[Bison](https://www.gnu.org/software/bison/) and
-[Flex](https://github.com/westes/flex) are used by executable semantics.
-Although we may
-[switch to a hemertic toolchain later](https://github.com/carbon-language/carbon-lang/issues/266),
-an install is currently required.
-
-Our recommended way of installing is:
-
-```bash
-brew install bison flex
-```
-
-On MacOS, it will be necessary to explicitly add the installed paths to the
-`PATH` environment variable so that the brew-installed versions are used instead
-of Xcode-installed versions. Read `brew` output for instructions.
-
 ### buildifier
 
 [Buildifier](https://github.com/bazelbuild/buildtools/tree/master/buildifier) is