samples: net: google_iot_mqtt: Fix CI for the sample

The sample did not specify any tests in its sample.yaml file, causing CI
failures.

Additionally, the sample requires to generate secure key before building
the sample, otherwise the build fails. For the CI needs, specify a dummy
test key, which is only included if the build was triggered by Twister.
For this, specify a USE_DUMMY_KEY variable in the sample.yaml file.

Finally, fix some minor build issues in the sample itself - remove
unused variable, and remove deleted (and also unneeded) mbed TLS header
inclusion.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>