pickfirstleaf: remove deprecated package (#9254)

The experimental and deprecated package contained aliases designed to
help users migrate to symbols of the same name in the `pickfirst`
package. These deprecated symbols have been in place since #8672, which
was included in the gRPC-Go 1.77 release.

RELEASE NOTES:
* pickfirstleaf: remove the deprecated
[`pickfirstleaf`](https://pkg.go.dev/google.golang.org/grpc@v1.82.1/balancer/pickfirst/pickfirstleaf)
package. Please use symbols with the same name from the
[`pickfirst`](https://pkg.go.dev/google.golang.org/grpc@v1.82.1/balancer/pickfirst)
package instead.