[mlir][python][cmake] Remove unsupported argument from AddMLIRPython. (#123858)

See https://cmake.org/cmake/help/latest/policy/CMP0175.html

> The `OUTPUT` form does not accept `PRE_BUILD`, `PRE_LINK`, or
`POST_BUILD` keywords.

When using CMake version 3.31+, this results in ~2000 lines of warning
spam in my downstream project:

```
CMake Warning (dev) at build-gcc/lib/cmake/mlir/AddMLIRPython.cmake:606 (add_custom_command):
  The following keywords are not supported when using
  add_custom_command(OUTPUT): PRE_BUILD.

  Policy CMP0175 is not set: add_custom_command() rejects invalid arguments.
  Run "cmake --help-policy CMP0175" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.
Call Stack (most recent call first):
  build-gcc/lib/cmake/mlir/AddMLIRPython.cmake:222 (add_mlir_python_sources_target)
  build-gcc/lib/cmake/mlir/AddMLIRPython.cmake:256 (_process_target)
  compiler/bindings/python/CMakeLists.txt:239 (add_mlir_python_modules)
This warning is for project developers.  Use -Wno-dev to suppress it.
```

General docs:
https://cmake.org/cmake/help/latest/command/add_custom_command.html.
Note that `PRE_BUILD` only appears in the _second_ signature for the
function (which takes `TARGET`) not the first (which takes `OUTPUT`).

0 files changed, 0 insertions, 0 deletions