Remove NodePrometheusRequestBuilder class

This class is no longer needed because of refactoring that happened in the upstream in the past.

Closes: #99

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Some details:

1. First the method `Action.newRequestBuilder(client)` was removed from Elasticsearch (see: elastic/elasticsearch#30966, specifically here elastic/elasticsearch@46e8d97#diff-34311f69b4f5ea90a3e88cabb8d3167f6c0651e5172f906750efc8a8276e4320L37).

2. Then it was removed from the Elasticsearch Prometheus exporter NodePrometheusMetricsAction class (see: vvanholl/elasticsearch-prometheus-exporter@deb496d#diff-d4bd29c3b4f015bf843e60db39c318a2560c6d9384942bbb271ed662ecf0d507L41).

Signed-off-by: Lukáš Vlček <lukas.vlcek@aiven.io>