handle: remove has_sub

It is only used for the C api and the FIXME there is not useful
since we have to turn it into gboolean anyway
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latest
Status Job ID Name Coverage
  Test
passed #188748
fedora test

00:07:57

 
  Lint
manual #188750
allowed to fail manual
clippy
passed #188749
rustfmt

00:00:45

passed #188751
tarball

00:10:38

 
  Cross Distro
passed #188754
debian_i386:testing

00:09:35

passed #188753
debian:testing

00:08:23

passed #188752
fedora:rawhide

00:08:19