remove useless methods
[ci skip] remove references to `Numeric#from_now`
Merge pull request #17892 from y-yagi/active_support_core_extensions_guide

[ci skip] remove references to `Numeric#from_now`
Merge pull request #17890 from sergey-alekseev/remove-useless-methods

remove useless methods
skip test to get `sqlite3_mem` suite passing.
document that `.delete` does work on `#readonly?` records. Closes #11860

[ci skip]

This is due to the fact that `.delete` is directly translated to SQL.
It tries to follow the same rules as `.delete_all` which is not able
to verify that records are `#readonly?`.