Undeprecate name_prefix for ssl_certificate.

As discussed in hashicorp#1326, we're not going to remove name_prefix for
compute_ssl_certificate, because it makes the common use case more
ergonomic by a good amount, and the only cost is it's harder to maintain
the autogenerated code, and we've decided the benefits outweigh the
costs in this circumstance.

Author: paddycarver

docs/r/compute_ssl_certificate.html.markdown

@@ -70,7 +70,7 @@ The following arguments are supported:
 * `name` - (Optional) A unique name for the SSL certificate. If you leave
   this blank, Terraform will auto-generate a unique name.
 
-* `name_prefix` - (Deprecated, Optional) Creates a unique name beginning with the specified
+* `name_prefix` - (Optional) Creates a unique name beginning with the specified
   prefix. Conflicts with `name`.
 
 * `description` - (Optional) An optional description of this resource.