HADOOP-18889. Third party storage followup. (#6186)

Followup to HADOOP-18889 third party store support;

Fix some minor review comments which came in after the merge.

hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/S3AFileSystem.java

@@ -1357,7 +1357,7 @@ public class S3AFileSystem extends FileSystem implements StreamCapabilities,
     public String getBucketLocation(String bucketName) throws IOException {
       final String region = trackDurationAndSpan(
           STORE_EXISTS_PROBE, bucketName, null, () ->
-              once("getBucketLocation()", bucketName, () ->
+              invoker.retry("getBucketLocation()", bucketName, true, () ->
                   // If accessPoint then region is known from Arn
                   accessPoint != null
                       ? accessPoint.getRegion()

hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/auth/SignerFactory.java

@@ -81,16 +81,6 @@ public final class SignerFactory {
     SIGNERS.put(signerType, signerClass);
   }
 
-  /**
-   * Check if the signer has already been registered.
-   * @param signerType signer to get
-   * @throws IllegalArgumentException if the signer type is unknown.
-   */
-  public static void verifySignerRegistered(String signerType) {
-    checkArgument(isSignerRegistered(signerType),
-        "unknown signer type: %s", signerType);
-  }
-
   /**
    * Check if the signer has already been registered.
    * @param signerType signer to get