Remove discovery-file plugin (#33257)

In #33241 we moved the file-based discovery functionality to core
Elasticsearch, but preserved the `discovery-file` plugin, and support for the
existing location of the `unicast_hosts.txt` file, for BWC reasons. This commit
completes the removal of this plugin.

Author: DaveCTurner

docs/plugins/discovery-file.asciidoc

@@ -21,10 +21,6 @@ The Azure Classic discovery plugin uses the Azure Classic API for unicast discov
 
 The Google Compute Engine discovery plugin uses the GCE API for unicast discovery.
 
-<<discovery-file,File-based discovery>>::
-
-The File-based discovery plugin allows providing the unicast hosts list through a dynamically updatable file.
-
 [float]
 ==== Community contributed discovery plugins
 
@@ -38,5 +34,3 @@ include::discovery-ec2.asciidoc[]
 include::discovery-azure-classic.asciidoc[]
 
 include::discovery-gce.asciidoc[]
-
-include::discovery-file.asciidoc[]

docs/plugins/discovery.asciidoc

@@ -23,7 +23,6 @@ U7321H6 analysis-stempel        {version} The Stempel (Polish) Analysis plugin i
 U7321H6 analysis-ukrainian      {version} The Ukrainian Analysis plugin integrates the Lucene UkrainianMorfologikAnalyzer into elasticsearch.
 U7321H6 discovery-azure-classic {version} The Azure Classic Discovery plugin allows to use Azure Classic API for the unicast discovery mechanism
 U7321H6 discovery-ec2           {version} The EC2 discovery plugin allows to use AWS API for the unicast discovery mechanism.
-U7321H6 discovery-file          {version} Discovery file plugin enables unicast discovery from hosts stored in a file.
 U7321H6 discovery-gce           {version} The Google Compute Engine (GCE) Discovery plugin allows to use GCE API for the unicast discovery mechanism.
 U7321H6 ingest-attachment       {version} Ingest processor that uses Apache Tika to extract contents
 U7321H6 ingest-geoip            {version} Ingest processor that uses looksup geo data based on ip adresses using the Maxmind geo database

docs/reference/cat/plugins.asciidoc

@@ -22,4 +22,12 @@ See {plugins}/repository-gcs-client.html#repository-gcs-client[Google Cloud Stor
 ==== Analysis Plugin changes
 
 * The misspelled helper method `requriesAnalysisSettings(AnalyzerProvider<T> provider)` has been
-renamed to `requiresAnalysisSettings`
+renamed to `requiresAnalysisSettings`
+
+==== File-based discovery plugin
+
+* This plugin has been removed since its functionality is now part of
+Elasticsearch and requires no plugin. The location of the hosts file has moved
+from `$ES_PATH_CONF/file-discovery/unicast_hosts.txt` to
+`$ES_PATH_CONF/unicast_hosts.txt`. See <<file-based-hosts-provider, the
+file-based hosts provider documentation>> for further information.