Remove sk_float_rsqrt_portable

It is both unused and redundant (same impl as regular sk_float_rsqrt).

Change-Id: If82deeb5ff17525906da27be1905509c90b95306
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1170116
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>