scripts: twister: drop support for space-separated lists

Support for space-separated lists was deprecated a long time ago
a91620f5 so it is time to remove
support for them.

Any project that has not migrated can still use
`scripts/utils/twister_to_list.py` to automatically migrate twister
files.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>