[SPARK-52291] Remove nightly info from README.md

### What changes were proposed in this pull request?

This PR aims to remove `nightly` info from `README.md`.

### Why are the changes needed?

- In `Install Helm Chart` section, the developer can build and install it already.
- For the published one, we had better focus on the official one in `README.md` because it's the landing page in GitHub.

### Does this PR introduce _any_ user-facing change?

No. This is a documentation improvement.

### How was this patch tested?

Manual review.

### Was this patch authored or co-authored using generative AI tooling?

No

Closes apache#223 from dongjoon-hyun/SPARK-52291.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

README.md

@@ -130,15 +130,6 @@ $ kubectl delete sparkapp pi-on-yunikorn
 sparkapplication.spark.apache.org "pi-on-yunikorn" deleted
 ```
 
-## Try nightly build for testing
-
-As of now, you can try `spark-kubernetes-operator` nightly version in the following way.
-
-```
-$ helm install spark-kubernetes-operator \
-https://nightlies.apache.org/spark/charts/spark-kubernetes-operator-0.3.0-SNAPSHOT.tgz
-```
-
 ## Clean Up
 
 Check the existing Spark applications and clusters. If exists, delete them.
@@ -161,11 +152,6 @@ $ kubectl delete crd sparkapplications.spark.apache.org
 $ kubectl delete crd sparkclusters.spark.apache.org
 ```
 
-In case of nightly builds, remove the snapshot image.
-```
-$ docker rmi apache/spark-kubernetes-operator:main-snapshot
-```
-
 ## Contributing
 
 Please review the [Contribution to Spark guide](https://spark.apache.org/contributing.html)