[2.1.10] FreeBSD: Remove extra arc_reduce_target_size() call

[amotin]

Remove arc_reduce_target_size() call from arc_prune_task(). The idea of arc_prune_task() is to remove external references on ARC metadata, such as vnodes. Since arc_prune_async() is called only from ARC itself, it makes no sense to create a parasitic loop between ARC eviction and the pruning, treatening to drop ARC to its minimum. I can't guess why it was added as part of FreeBSD to OpenZFS integration.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.