Remove deprecated commands (#14726)

# Description

Remove commands which were deprecated in 0.101:

* `split-by` (#14019)
* `date to-record` and `date to-table` (#14319)

# User-Facing Changes

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

TODO: `grep` (`ag`) doc repo for any usage of these commands

crates/nu-command/tests/commands/mod.rs

@@ -103,7 +103,6 @@ mod skip;
 mod sort;
 mod sort_by;
 mod source_env;
-mod split_by;
 mod split_column;
 mod split_row;
 mod str_;