[flang] [llvm] [Flang] Remove FLANG_INCLUDE_RUNTIME (PR #124126)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `flang-runtime-cuda-clang` running on `as-builder-7` while building `flang-rt,flang,llvm` at step 9 "cmake-configure-flang-runtime".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/7/builds/12751

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 9 (cmake-configure-flang-runtime) failure: cmake (failure)
CMake Error: The source directory "/home/buildbot/worker/as-builder-7/ramdisk/flang-runtime-cuda-clang/llvm-project/flang/runtime" does not exist.
Specify --help for usage, or press the help button on the CMake GUI.

```

</details>

https://github.com/llvm/llvm-project/pull/124126