Remove handleValidationErrors and handleParseErrors options from useMaskedErrors

[n1ru4l]

useMaskedErrors should be scoped to the execution and context creation.

Masking validation errors ONLY makes sense when introspection is disabled.

ONLY masking validation errors OR ONLY disabling introspection errors does not make sense, as both can be abused for reverse-engineering the GraphQL schema (see https://github.com/nikitastupin/clairvoyance for reverse-engineering the schema based on validation error suggestions).

Thus the best solution here is to actually create a separate plugin that does both, disabling introspection and hiding validation errors.

In terms of masking/hiding parse errors, I am not sure whether this is really needed.