Toolchain: Remove git clone of project from Dockerfile

The goal of these more recent additions to the Dockerfile is to provide
a working copy of SerenityOS with the toolchain prebuilt. To me, these
additions feel misplaced:

- The toolchain is built assuming the i686 architecture, which may not
  be what you want.
- You get a shallow clone of the project limiting you in your abilities
  to navigate through the project's history or bisect.
- There's this awkward directory structure of `/serenity/serenity-git`
  and `/serenity/out`.

The Dockerfile is immensely useful for building SerenityOS in a
containerized environment, separate from the host's environment. If we
want to automate builds, we can always use CI or extend this image to
do so. For now, let's remove the `git clone` and associated actions.

Fixes #9310.

Toolchain/Dockerfile

@@ -29,10 +29,3 @@ RUN apt-get update -y \
         unzip \
     && rm -rf /var/lib/apt/lists/ \
     && update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-10 900 --slave /usr/bin/g++ g++ /usr/bin/g++-10
-
-RUN git clone --depth 1 https://github.com/SerenityOS/serenity.git /serenity/serenity-git
-RUN cd /serenity/serenity-git/Toolchain; \
-        ./BuildIt.sh
-
-WORKDIR /serenity
-VOLUME /serenity/out