pf: Remove unused return value from (de)hook_pf()

These functions always return 0, which is good, because the code calling
them doesn't handle this error gracefully.

As the functions always succeed remove their return value, and the code
handling their errors (because it was never executed anyway).

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC (“Netgate”’)

(cherry picked from commit 8a439f324e9010a122fa4c00426bde70dc373c2f)