refactor(register): remove deprecated register method

[gforsyth]

refactor(register): remove deprecated register method

BREAKING CHANGE: The deprecated register method has been removed.
Please use the file-specific read_* methods instead. For in-memory
objects, pass them to ibis.memtable or create_table.

Description of changes

The register method is removed everywhere. Test files have been renamed to
test_io.py instead of test_register.py, or they have been removed entirely
because they are duplicates of tests in test_client.py or elsewhere.

Issues closed

#10175

[gforsyth]

Hmm, so the datafusion backend has an option which is unique to that backend, where we allow a user to pass in a dictionary of {table_name: file_to_load} -- at connection creation, we hand each of these files off to register to load all of the files as tables.

I recognize the convenience here, but it's also against our general push towards requiring explicit table creation.

Do we rip that out, too? Or leave in _register as a private method in the datafusion backend to handle this?