iavf: Use the common RSS key without options RSS

[kbowling]

Summary

Use rss_getkey() when programming the RSS key through virtchnl regardless
of options RSS, matching the register-programming path.  The common key
is available independently of RSS queue placement; the driver-specific
default otherwise makes the chosen key depend on the PF's configuration
interface.

Remove the unused default-key helper and its declarations.  Preserve the
negotiated key length, zero padding, hash selections, and indirection-table
policy.

MFC after:      2 weeks
Sponsored by:   BBOX.io