fix(mega_linter): remove non-mandatory MegaLinter configuration variables

Since environment variables has higher precedence than settings from the MegaLinter configuration file, it's not possible to use MegaLinter locally with the same configuration without having things configured in multiple files. E.g. it's not possible to define `DISABLE_LINTERS` in `.mega-linter.yml` as long as it's set via environment variables.

BREAKING-CHANGE: remove job variables `FILTER_REGEX_EXCLUDE`, `VALIDATE_ALL_CODEBASE` and `DISABLE_LINTERS`