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GH-37532: [CI][Docs][MATLAB] Remove `GoogleTest` support from the CMake build system for the MATLAB interface (#37784)

### Rationale for this change

This pull request removes `GoogleTest` support from the CMake build system for the MATLAB interface.

1. `GoogleTest` support adds a lot of additional complexity to the CMake build system for the MATLAB interface, and we currently don't have any standalone C++ tests for the MATLAB interface code.
2. In order to use `GoogleTest` in the MATLAB CI workflows, we are currently relying on building the tests for the Arrow C++ libraries in order to "re-use" the `GoogleTest binaries. This adds additional overhead to the MATLAB CI workflows.
3. If we want to test some internal C++ code for the MATLAB interface in the future, we can instead use a MEX function to call the code from a MATLAB test as suggested by @ kou in https://github.com/apache/arrow/issues/37532#issue-1879320217.
4. There is [precedent for testing internal C++ code without GoogleTest for the Python bindings](https://github.com/apache/arrow/pull/14117).
5. On a somewhat related note - removing `GoogleTest` support will help unblock https://github.com/apache/arrow/pull/37773 as discussed in https://github.com/apache/arrow/pull/37773#issuecomment-1724207859.

### What changes are included in this PR?

1. Removed the `MATLAB_BUILD_TESTS` flag from the CMake build system for the MATLAB interface since there are no longer any C++ tests for the MATLAB interface to build.
2. Updated the `matlab_build.sh` CI workflow script to avoid building the tests for the Arrow C++ libraries and to no longer call `ctest`.
3. Updated the `README.md` for the MATLAB interface to no longer mention building or running C++ tests.
4. Updated the design document for the MATLAB Interface to no longer mention `GoogleTest` since we may end up testing internal C++ code using MEX function calls from MATLAB instead.
5. Removed placeholder C++ test (i.e. `placeholder_test.cc`).

### Are these changes tested?

Yes.

The MATLAB CI workflow is passing on all platforms.

### Are there any user-facing changes?

Yes.

There are no longer any C++ tests for the MATLAB interface. The `MATLAB_BUILD_TESTS` flag has been removed from the CMake build system to reflect this change. If a user supplies a value for `MATLAB_BUILD_TESTS` when building the MATLAB interface, the flag will be ignored by CMake.

### Future Directions

1. Add more developer-focused documentation on how to test C++ code via MEX function calls from MATLAB.

### Notes

1. In the future, we can consider testing internal C++ code using MEX function calls from MATLAB tests as suggested by @ kou in https://github.com/apache/arrow/issues/37532#issue-1879320217. Currently, we don't have any C++ tests that need to be adapted to use this approach.
2. Thank you @ sgilmore10 for your help with this pull request!
* Closes: #37532

Lead-authored-by: Kevin Gurney <kgurney@mathworks.com>
Co-authored-by: Sarah Gilmore <sgilmore@mathworks.com>
Signed-off-by: Kevin Gurney <kgurney@mathworks.com>