SI-6441 removes Symbol.isOverride

`Symbol.isOverride` joins `Symbol.isLocal` in the unfortunate family
of methods for which we had to break source compatibility because of
their utter brokenness.

Apparently, `isOverride` only returns true for those symbols that have
the OVERRIDE flag set (i.e. the ones that are derived from source definitions
that have the `override` modifier specified next to them). Of course,
this is very confusing, and that’s exacerbated by the fact that we can’t
fix this behavior, because there’s `internal#Symbol.isOverride` that someone
might rely on.

Given that there’s perfectly working `Symbol.allOverriddenSymbols`,
this commit removes the `Symbol.isOverride` API altogether.