Remove deprecated schema- and namespace-related APIs

[morozov]

Q	A
Type	improvement
BC Break	yes

See #4503.

[greg0ire]

Approving in case the answer to my question is no.

[greg0ire]

Should these also become final?

[morozov]

The purpose of this API is not really clear to me: there's a single use case which uses only the first element of the path:

dbal/src/Schema/PostgreSQLSchemaManager.php

Lines 177 to 180 in d19875a

	protected function _getPortableTableDefinition(array $table): string
	{
	$schemas = $this->getExistingSchemaSearchPaths();
	$firstSchema = array_shift($schemas);

So I'd expect some more rework here in the future.

But yeah, final for the wrapper makes sense regardless. I'd leave the "determine" method as is (in case if somebody hacks their own logic in there).