Remove deprecated 'gateway' settings used to defer cluster recovery pending a certain number of master nodes

[tlfeng]

Is your feature request related to a problem? Please describe.
Settings used to defer cluster recovery pending a certain number of master nodes are deprecated in Elasticsearch 7.7.

The following cluster settings are deprecated:

    gateway.expected_nodes
    gateway.expected_master_nodes
    gateway.recover_after_nodes
    gateway.recover_after_master_nodes

It is safe to recover the cluster as soon as a majority of master-eligible nodes have joined. There is no benefit in waiting for any additional master-eligible nodes to start.

To avoid deprecation warnings, discontinue use of the deprecated settings. If needed, use gateway.expected_data_nodes or gateway.recover_after_data_nodes to defer cluster recovery pending a certain number of data nodes.

Reference:
https://www.elastic.co/guide/en/elasticsearch/reference/7.10/breaking-changes-7.7.html#deprecate-defer-cluster-recovery-settings

Describe the solution you'd like
Remove the 4 deprecated cluster settings.

Describe alternatives you've considered
none.

Additional context
Mentioned in issue for master terminology change #472
A part of the issue for deprecated code removal #2773