[C++] FindArrowDataset requires Parquet

Parquet is an optional dependency of the dataset layer, but it appears that `FindArrowDataset.cmake` requires Parquet unconditionally.

Example:
https://app.travis-ci.com/github/apache/arrow/jobs/548648734#L3513

```Java

Could NOT find Parquet (missing: PARQUET_INCLUDE_DIR PARQUET_LIB_DIR

PARQUET_SO_VERSION)

Call Stack (most recent call first):

/usr/share/cmake-3.16/Modules/FindPackageHandleStandardArgs.cmake:393 (_FPHSA_FAILURE_MESSAGE)

cmake_modules/FindParquet.cmake:115 (find_package_handle_standard_args)

cmake_modules/FindArrowDataset.cmake:47 (find_package)

CMakeLists.txt:432 (find_package)
```

**Reporter**: [Antoine Pitrou](https://issues.apache.org/jira/browse/ARROW-14793) / @pitrou
#### Related issues:
- [[C++] CMake's find_package(Parquet) does not find Parquet with Arrow 3.0.0](https://github.com/apache/arrow/issues/27996) (is related to)

_{**Note**: *This issue was originally created as [ARROW-14793](https://issues.apache.org/jira/browse/ARROW-14793). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Inspect cmake_modules/FindArrowDataset.cmake around its Parquet lookup, then compare it with cmake_modules/FindParquet.cmake and the find_package call in CMakeLists.txt around line 432. Reproduce the dataset-layer configuration from the linked Travis job without Parquet; done means configuration no longer requires Parquet unconditionally.

Written by the indexing model from the issue text.