[NFC] Remove the llvm-external-projects/iree-dialects directory (#24466) (#24612)

## Overview

Removes the `llvm-external-projects/iree-dialects` directory now that
nothing in the compiler or runtime depends on it.

This is the final step of #24466. The production dependencies were
retired in #24543 and predecessors:
- the softmax matcher was reimplemented natively in
`GlobalOptimization/RaiseSpecialOps`,
- `ErrorCheckingTrackingListener` was moved into `Codegen/Common/`,
- `DropSchedulePass` was moved into `Codegen/Common/`,
- the callback-matcher transform ops
(`transform.iree.register_match_callbacks` / `match_callback` /
`take_first` / `emit_remark`) and their tests were dropped.

## Changes

- Delete `llvm-external-projects/iree-dialects/`.
- Drop the `iree_llvm_add_external_project(mlir-iree-dialects ...)` call
from the top-level `CMakeLists.txt`.
- Remove the dead iree-dialects target conversion from `bazel_to_cmake`.
- Drop the `IREEDialects*` Python sources + include path from
`compiler/bindings/python/CMakeLists.txt`, the `_ireeDialects`
`CMakeExtension` from `compiler/setup.py`, and the `iree_dialects.rst`
API doc.
- Remove the `iree-dialects-doc` build step from the website doc
generator.
- Remove the "Test iree-dialects" CI steps and the `check-iree-dialects`
targets from the ASan/UBSan scripts (and the now-unused
`test_iree_dialects.sh`).

## Testing

- **Local (Windows + MSVC, Ninja, Release):** CMake configure is clean
after the external-project removal only `stablehlo` remains as an
external LLVM project and a full-tree grep finds zero dangling
references. The full `iree-opt` target builds, links, and runs
(`iree-opt --version` → LLVM 23.0.0git).

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Signed-off-by: Alex-Wengg <hanweng9@gmail.com>