Remove top stack frame from getCurrentStack (#30306)

The full stack is the current execution stack (`new Error().stack`) +
the current owner stack (`React.captureOwnerStack()`).

The idea with the top frame was that when we append it to console.error
we'd include both since otherwise the true reason would be obscured
behind the little `>` to expand. So we'd just put both stack front and
center. By adding this into getCurrentStack it was easy to use the same
filtering. I never implemented in Fizz or Flight though.

However, with the public API `React.captureOwnerStack()` it's not
necessary to include the current stack since you already have it and
you'd have filtering capabilities in user space too.

Since I'm removing the component stacks from React itself we no longer
need this. It's expected that maybe RDT or framework polyfill would
include this same technique though.