Fix leftover mentions of method parameter in Percentile Aggs (#51272)

The method parameter is not used in the percentile aggs, instead
the method is determined by the presence of `hdr` or `tdigest`
objects.

Relates to #8324

docs/reference/aggregations/metrics/percentile-rank-aggregation.asciidoc

@@ -183,7 +183,7 @@ number of significant digits). This means that if data is recorded with values f
 microseconds) in a histogram set to 3 significant digits, it will maintain a value resolution of 1 microsecond for values up to
 1 millisecond and 3.6 seconds (or better) for the maximum tracked value (1 hour).
 
-The HDR Histogram can be used by specifying the `method` parameter in the request:
+The HDR Histogram can be used by specifying the `hdr` object in the request:
 
 [source,console]
 --------------------------------------------------

server/src/main/java/org/elasticsearch/search/aggregations/metrics/PercentilesAggregationBuilder.java

@@ -49,7 +49,6 @@ public class PercentilesAggregationBuilder extends LeafOnly<ValuesSource, Percen
     private static final double[] DEFAULT_PERCENTS = new double[] { 1, 5, 25, 50, 75, 95, 99 };
     public static final ParseField PERCENTS_FIELD = new ParseField("percents");
     public static final ParseField KEYED_FIELD = new ParseField("keyed");
-    public static final ParseField METHOD_FIELD = new ParseField("method");
     public static final ParseField COMPRESSION_FIELD = new ParseField("compression");
     public static final ParseField NUMBER_SIGNIFICANT_DIGITS_FIELD = new ParseField("number_of_significant_value_digits");