[clang] [libclang/python] Remove CompletionString.AvailabilityKindCompat (PR #210682)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-flang-rhel-clang` running on `ppc64le-flang-rhel-test` while building `clang` at step 6 "test-build-unified-tree-check-flang".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/48700

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

```
Step 6 (test-build-unified-tree-check-flang) failure: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
...
PASS: Flang :: Transforms/dlti-dependency.fir (4806 of 4816)
PASS: Flang :: Transforms/debug-omp-target-op-1.fir (4807 of 4816)
PASS: Flang :: Transforms/verify-known-runtime-functions.fir (4808 of 4816)
PASS: Flang :: Lower/OpenMP/rtl-flags.f90 (4809 of 4816)
PASS: Flang :: Transforms/debug-dwarf-version.fir (4810 of 4816)
PASS: Flang :: Intrinsics/math-codegen.fir (4811 of 4816)
PASS: Flang :: Transforms/OpenACC/acc-implicit-data-fortran.F90 (4812 of 4816)
PASS: Flang :: Transforms/debug-line-table.fir (4813 of 4816)
PASS: Flang :: Driver/fopenmp.f90 (4814 of 4816)
PASS: Flang :: Driver/linker-options.f90 (4815 of 4816)
command timed out: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=2258.846339

```

</details>

https://github.com/llvm/llvm-project/pull/210682