Remove redundant argument from POSIX_SOCKET_FAMILY_REGISTER macro

Remove redundant argument from POSIX_SOCKET_FAMILY_REGISTER macro

In the `POSIX_SOCKET_FAMILY_REGISTER` macro, when the IPv6 option is
selected, the third argument produces a compilation error.

Due to the macro being defined in
`unikraft/lib/posix-socket/include/uk/socket_driver.h` with only 2
arguments, the third argument becomes redundant.

This commit removes it and ensures that the `AF_INET6` is successfully
registered.

Signed-off-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Reviewed-by: Razvan Deaconescu <razvan.deaconescu@cs.pub.ro>
Approved-by: Marc Rittinghaus <marc.rittinghaus@kit.edu>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Pull-Request: #18