queries: Fix `comment.unused` highlights in Erlang

* Other languages use `comment.unused` instead of `comment.discard`.
* Fix the precedence of discarded variables - previously the parameter
  highlight was higher precedence so discarded variables in the
  parameter list were highlighted as parameters instead of
  `comment.unused`.

runtime/queries/erlang/highlights.scm

@@ -1,5 +1,5 @@
 ; Comments
-(tripledot) @comment.discard
+(tripledot) @comment.unused
 
 [(comment) (line_comment) (shebang)] @comment
 
@@ -114,10 +114,6 @@
   ] @comment.block.documentation)
  (#any-of? @keyword "doc" "moduledoc"))
 
-; Ignored variables
-((variable) @comment.discard
- (#match? @comment.discard "^_"))
-
 ; Macros
 (macro
   "?"+ @keyword.directive
@@ -163,3 +159,7 @@
 
 (record field: (atom) @variable.other.member)
 (record name: (atom) @type)
+
+; Ignored variables
+((variable) @comment.unused
+ (#match? @comment.unused "^_"))