Fix bug that prevents selected join field to be used (#3213)

Remove setting join field even if it is already set.

Signed-off-by: Vijayan Balasubramanian <balasvij@amazon.com>

Signed-off-by: Vijayan Balasubramanian <balasvij@amazon.com>
(cherry picked from commit 8c6f569)
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

# Conflicts:
#	CHANGELOG.md

src/plugins/region_map/public/region_map_visualization.js

@@ -158,7 +158,6 @@ export function createRegionMapVisualization({
       let selectedLayer;
       if (DEFAULT_MAP_CHOICE === this._params.layerChosenByUser && this._params.selectedLayer) {
         selectedLayer = await this._loadConfig(this._params.selectedLayer);
-        this._params.selectedJoinField = selectedLayer?.fields[0];
       } else if (
         CUSTOM_MAP_CHOICE === this._params.layerChosenByUser &&
         this._params.selectedCustomLayer