Revert "default: fork platform_testing repo"

This reverts commit 57b54e42.

The AP2A (QPR2) release is causing us too much issues:
new lunch targets, different CTS/VTS behaviour,
4k playback regression, ...

Since we will be updating to UQ1A.240205.004.B1 instead, we no longer
need this fork.

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>