docs: rust: remove `CC=clang` mentions

Nowadays all architectures except s390 recommend using `LLVM=1` instead of
`CC=clang`, and since commit a3c6bfba4429 ("Documentation/llvm: refresh
docs") the Kbuild LLVM documentation makes `LLVM=1` the way to go:

    We want to encourage the use of ``LLVM=1`` rather than just
    ``CC=clang``. Make that suggestion "above the fold" and "front and
    center" in our docs.

In particular, that commit removes the examples with `CC=clang`.

Thus do the same in the Rust Quick Start guide, i.e. remove the `CC=clang`
mentions, especially since the architectures that have had their Rust
support upstreamed (or soon to be upstreamed) are all `LLVM=1` ones.
And perhaps by the time Rust is supported for s390 (or new architectures),
it may have moved to `LLVM=1` anyway. Otherwise, this can be added back
if needed (or perhaps an extra link to Documentation/kbuild/llvm.rst).

This should also help avoiding potential confusion around `CC=clang` [1].

Link: https://lore.kernel.org/rust-for-linux/6df6e8e5-8d5b-4d3d-91b5-bc0e90c424ea@nvidia.com/ [1]
Reviewed-by: Ariel Miculas <amiculas@cisco.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Link: https://lore.kernel.org/r/20231215124751.175191-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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