[chore] [exporter/elasticsearch] remove defunct config (#33803)

**Description:**

Remove exporter/elasticsearch config fields & mentions in the README.
Nothing has ever used this config.
They were added in
#2324,
but were not used.

If one _were_ to add fields to all documents, then I think that should
be done with a processor.

**Link to tracking Issue:**

N/A

**Testing:**

N/A

**Documentation:**

N/A

Author: axw

.chloggen/elasticsearch-rm-mapping-fields-file.yaml

@@ -0,0 +1,30 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: breaking
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: elasticsearchexporter
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Remove defunct "file" and "fields" configuration settings.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [33803]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+  This is a breaking change only because removing the attributes would prevent
+  collector startup if those attributes are specified, but otherwise there is
+  no functional change. These configuration attributes have never done anything.
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]

exporter/elasticsearchexporter/README.md

@@ -129,8 +129,6 @@ behaviours, which may be configured through the following settings:
     - `raw`: Omit the `Attributes.` string prefixed to field names for log and 
              span attributes as well as omit the `Events.` string prefixed to
              field names for span events. 
-  - `fields` (optional): Configure additional fields mappings.
-  - `file` (optional): Read additional field mappings from the provided YAML file.
   - `dedup` (default=true; DEPRECATED, in future deduplication will always be enabled):
     Try to find and remove duplicate fields/attributes from events before publishing
     to Elasticsearch. Some structured logging libraries can produce duplicate fields

exporter/elasticsearchexporter/config.go

@@ -160,12 +160,6 @@ type MappingsSettings struct {
 	// Mode configures the field mappings.
 	Mode string `mapstructure:"mode"`
 
-	// Additional field mappings.
-	Fields map[string]string `mapstructure:"fields"`
-
-	// File to read additional fields mappings from.
-	File string `mapstructure:"file"`
-
 	// Try to find and remove duplicate fields
 	//
 	// Deprecated: [v0.104.0] deduplication will always be applied in future,