twister: allow using west-flash with 'erase'

Using `erase` with west-flash was blocked as it was
messing with sysbuild. With #69748 the issue is fixed, hence
'erase' is no longer blocked.
Remove obsolete twister test

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>
2 files changed