panic loop when removing slog device

There is a window in the slog removal code where a panic loop could
ensue if the system crashes during that operation. The original design
of slog removal did not persisted any state because the removal happened
synchronously. This was changed by a later commit which persisted the
vdev_removing flag and exposed this bug. If a slog removal is in
progress and happens to crash after persisting the vdev_removing flag to
the label but before the vdev is removed from the spa config, then the
pool will continue to panic on import. Here's a sample of the panic:

[ 134.387411] VERIFY0(0 == dmu_buf_hold_array(os, object, offset, size,
FALSE, FTAG, &numbufs, &dbp)) failed (0 == 22)
[ 134.393865] PANIC at dmu.c:1135:dmu_write()
[ 134.396035] Kernel panic - not syncing: VERIFY0(0 ==
dmu_buf_hold_array(os, object, offset, size, FALSE, FTAG, &numbufs,
&dbp)) failed (0 == 22)
[ 134.397857] CPU: 2 PID: 5914 Comm: txg_sync Kdump: loaded Tainted:
P OE 5.4.0-1100-dx2023020205-b3751f8c2-azure #106
[ 134.407938] Hardware name: Microsoft Corporation Virtual
Machine/Virtual Machine, BIOS 090008 12/07/2018
[ 134.407938] Call Trace:
[ 134.407938] dump_stack+0x57/0x6d
[ 134.407938] panic+0xfb/0x2d7
[ 134.407938] spl_panic+0xcf/0x102 [spl]
[ 134.407938] ? traverse_impl+0x1ca/0x420 [zfs]
[ 134.407938] ? dmu_object_alloc_impl+0x3b4/0x3c0 [zfs]
[ 134.407938] ? dnode_hold+0x1b/0x20 [zfs]
[ 134.407938] dmu_write+0xc3/0xd0 [zfs]
[ 134.407938] ? space_map_alloc+0x55/0x80 [zfs]
[ 134.407938] metaslab_sync+0x61a/0x830 [zfs]
[ 134.407938] ? queued_spin_unlock+0x9/0x10 [zfs]
[ 134.407938] vdev_sync+0x72/0x190 [zfs]
[ 134.407938] spa_sync_iterate_to_convergence+0x160/0x250 [zfs]
[ 134.407938] spa_sync+0x2f7/0x670 [zfs]
[ 134.407938] txg_sync_thread+0x22d/0x2d0 [zfs]
[ 134.407938] ? txg_dispatch_callbacks+0xf0/0xf0 [zfs]
[ 134.407938] thread_generic_wrapper+0x83/0xa0 [spl]
[ 134.407938] kthread+0x104/0x140
[ 134.407938] ? kasan_check_write.constprop.0+0x10/0x10 [spl]
[ 134.407938] ? kthread_park+0x90/0x90
[ 134.457802] ret_from_fork+0x1f/0x40

This change no longer persists the vdev_removing flag when removing slog
devices and also cleans up some code that was added which is not used.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Signed-off-by: George Wilson <gwilson@delphix.com>
Closes #14652