ESQL: Drop support for brackets from METADATA syntax (#119846)

This drops the deprecated support for having the METADATA clause
provided within square brakets.

Fixes #115401

Author: bpintea

docs/changelog/119846.yaml

@@ -0,0 +1,12 @@
+pr: 119846
+summary: Drop support for brackets from METADATA syntax
+area: ES|QL
+type: deprecation
+issues:
+ - 115401
+deprecation:
+  title: Drop support for brackets from METADATA syntax
+  area: ES|QL
+  details: Please describe the details of this change for the release notes. You can
+    use asciidoc.
+  impact: Please describe the impact of this change to users

x-pack/plugin/esql/src/main/antlr/EsqlBaseParser.g4

@@ -143,18 +143,9 @@ indexString
     ;
 
 metadata
-    : metadataOption
-    | deprecated_metadata
-    ;
-
-metadataOption
     : METADATA UNQUOTED_SOURCE (COMMA UNQUOTED_SOURCE)*
     ;
 
-deprecated_metadata
-    : OPENING_BRACKET metadataOption CLOSING_BRACKET
-    ;
-
 metricsCommand
     : DEV_METRICS indexPattern (COMMA indexPattern)* aggregates=aggFields? (BY grouping=fields)?
     ;