tc2: Removed TF-M tests

* TF-M tests are auto-fetched during TF-M build if external test path is not provided
* Default fetch is properly maintained with compatible version of TF-M and TF-M tests
* Hence, removing TF-M tests as external repo to avoid the maintenance.

Signed-off-by: Tintu Thomas <tintu.thomas@arm.com>
Change-Id: I2032b16946686b47d85db10cbec11ca5f362998e