keep the block in place when the anchor is removed while a queued track plays

`adjustAnchorOnRemove` moved the anchor to the canonical predecessor of the
removed track, which in a shuffled queue is an unrelated play position, so
removing the context anchor while playing a queued track teleported the
up-next block somewhere random (and out of view).

When the removed context track is the anchor, move the anchor to the track
before it in play order (or -1), so the block stays where it sits. This also
covers removing the playing context anchor, so the controller's separate
re-anchor is no longer needed and is removed.

app/src/test/kotlin/com/subtracks/playback/PlaybackControllerTest.kt

@@ -1670,6 +1670,32 @@ class PlaybackControllerTest {
         )
     }
 
+    @Test
+    fun removingTheAnchorWhilePlayingABlockTrackKeepsTheBlockInPlace() {
+        seedShuffledQueue(seed = 5L)
+        seedSong("x1", "al2")
+        controller.addToQueue(1, QueueKind.Song, "x1")
+        await { runBlocking { controller.upcomingItem()?.id } == "x1" }
+
+        controller.playAt(1)
+        await {
+            controller.state.value.item
+                ?.id == "x1"
+        }
+        val before = runBlocking { queues.snapshot() }
+
+        runBlocking { controller.removeAt(before.anchorPlay) }
+
+        val after = runBlocking { queues.snapshot() }
+        assertEquals(-1L, after.anchorPlay)
+        assertEquals("x1", runBlocking { queues.itemAt(after, 0)?.song?.id })
+        assertEquals(
+            "x1",
+            controller.state.value.item
+                ?.id,
+        )
+    }
+
     @Test
     fun removingThePlayingTrackWhileShuffledPlaysTheNextShuffledTrack() {
         seedShuffledQueue(seed = 12345L)