scripts: twister: Remove harness repeat field

Removed Harness's repeat field.

Added to schemas in e722db14,
together with the rest of harness_config.
Added to Harness when it was created.

Now present only in one sample -
mec15xxevb_assy6853 power management sample.

Could not find definitive proof whether it has ever been used.

Signed-off-by: Lukasz Mrugala <lukaszx.mrugala@intel.com>