Config Schema: Revert invalid "oneOf" based validation

[medikoo]

Fixes issues introduced with: #8309, #8114 and #8230

Idea was to require few properties exchangeably but also allow not provide any of them at all, via:

{ "oneOf": [
  { "required": [] },
  { "required": ["foo"] },
  { "required": ["bar"] }, 
] }

That can't work, because if either foo or bar is provided, two schemas out of listed are matched, and that is invalid against one of rule.

Without using sophisticated not constructs there seems no way to reliably validate this by schema, therefore removed it/

Closes: #8373