Remove FIXME about NLL diagnostic that is already improved
The FIXME was added in #46984 when the diagnostic message looked like this:
// FIXME(#46983): error message should be better
&s.0 //~ ERROR free region `` does not outlive free region `'static`
The message was improved in #90667 and now looks like this:
&s.0 //~ ERROR lifetime may not live long enough
but the FIXME was not removed. The issue #46983 about that diagnostics should be improved has been closed. We can remove the FIXME now.
(This PR was made for #44366.)
f44f0279fa