This commit removes the `compiler-rt` submodule from this repository.
The goal here is to align the `compiler-rt` used for compiling C
intrinsics with the upstream rust-lang/rust's usage of `llvm-project`.
Currently we have both an `llvm-project` repository as well as
`compiler-rt`, but they can naturally get out of sync and it's just one
more submodule to manage.

The thinking here is that the feature `c` for this crate, when
activated, will require the user to configure where the source code for
`compiler-rt` is present. This places the onus on the builder of
`compiler-builtins` to check-out and arrange for the appropriate
`compiler-rt` source code to be placed somewhere. For rust-lang/rust
this is already done with the `llvm-project` submodule, and we can
arrange for it to happen on this crate's CI anyway.

For users of this crate this is a bit of a bummer, but `c` is disabled
by default anyway and it seems unlikely that `c` is explicitly opted in
to all that much. (given the purpose of this crate)

This should allow us to archive the `compiler-rt` repository and simply
use `llvm-project` in the rust-lang/rust repository.