examples: Remove unused, invalid CTest cases

The test cases added by the examples' calls to `add_test()` are invalid.
For example, in the speech example, the commands

    pytest -s speech/tests

recursively runs every pytest script inside `speech/tests`, which does
not work, because some scripts are for AWS only and some are for Azure
only. Also, it misses arguments for AVH path and credentials path.

Remove those test cases, as they are not used anymore. In the CI script
`.gitlab-ci.yml`, we invoke one specific pytest script at a time and
correctly pass required arguments based on what the test is.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>