x/tools/gopls: redundant diagnostics due to quick fixes

The following code causes three "too many returns" diagnostics in gopls. Notably, the present but empty result list is required, so this is probably a low priority bug.

```go
func _() () {
  return 0
}
```

It looks like somehow the "fillreturns" and "noresultvalues" analyzers are colliding, and causing duplication. This may manifest in other ways as we add more quick fixes.