Correctly remove block decorations whose markers have been destroyed

In https://github.com/atom/atom/pull/15503 we mistakenly assumed
`marker.isValid` accounted only for the validity of the marker. However,
that method returns `false` also for markers that are valid but have
been destroyed. As a result, the editor component was mistakenly not
removing block decorations associated with such markers.

With this commit we will rely on the local `wasValid` variable instead.
If its value is `true`, it means that the block decoration has been
accounted for in the `lineTopIndex` and must, as a result, be cleaned up
in case the marker or the decoration gets destroyed.

src/text-editor-component.js

@@ -2506,7 +2506,7 @@ class TextEditorComponent {
         didUpdateDisposable.dispose()
         didDestroyDisposable.dispose()
 
-        if (marker.isValid()) {
+        if (wasValid) {
           this.blockDecorationsToMeasure.delete(decoration)
           this.heightsByBlockDecoration.delete(decoration)
           this.blockDecorationsByElement.delete(element)