fix(metadata): drop unused performer/plural-credit tag mappings

Per Copilot & Codex review on PR #5527:

- performercredit was registered as a plain string mapping, but PERFORMER
  is type:pair ('PERFORMER:instrument'). The credit form needs the same
  pair structure to be meaningful, and processPerformers doesn't read it
  either way. Better to remove the misleading mapping than ship dead
  config that suggests support we don't have.

- TagComposersCredit and TagLyricistsCredit (plural variants) were never
  consumed by any code path — only the singular TagComposerCredit and
  TagLyricistCredit are wired into roleMappings.

Both can be added back when the underlying feature is actually
implemented.

model/tag.go

@@ -216,11 +216,9 @@ const (
 	TagComposer           TagName = "composer"
 	TagComposerSort       TagName = "composersort"
 	TagComposerCredit     TagName = "composercredit"
-	TagComposersCredit    TagName = "composerscredit"
 	TagLyricist           TagName = "lyricist"
 	TagLyricistSort       TagName = "lyricistsort"
 	TagLyricistCredit     TagName = "lyricistcredit"
-	TagLyricistsCredit    TagName = "lyricistscredit"
 	TagDirector           TagName = "director"
 	TagDirectorCredit     TagName = "directorcredit"
 	TagProducer           TagName = "producer"
@@ -238,7 +236,9 @@ const (
 	TagArranger           TagName = "arranger"
 	TagArrangerCredit     TagName = "arrangercredit"
 	TagPerformer          TagName = "performer"
-	TagPerformerCredit    TagName = "performercredit"
+	// Performer credits are not currently surfaced: performer tags are
+	// `type: pair` ('PERFORMER:instrument') and the matching credit
+	// representation hasn't been designed yet. Tracked separately.
 
 	// ReplayGain
 

resources/mappings.yaml

@@ -58,16 +58,12 @@ main:
     aliases: [ tsoc, txxx:composersort, composersort, soco, wm/composersortorder ]
   composercredit:
     aliases: [ composer_credit, composercredit ]
-  composerscredit:
-    aliases: [ composers_credit, composerscredit ]
   lyricist:
     aliases: [ text, lyricist, ----:com.apple.itunes:lyricist, wm/writer ]
   lyricistsort:
     aliases: [ lyricistsort ]
   lyricistcredit:
     aliases: [ lyricist_credit, lyricistcredit ]
-  lyricistscredit:
-    aliases: [ lyricists_credit, lyricistscredit ]
   conductor:
     aliases: [ tpe3, conductor, ----:com.apple.itunes:conductor, wm/conductor ]
   conductorcredit:
@@ -225,8 +221,6 @@ main:
   performer:
     aliases: [performer]
     type: pair
-  performercredit:
-    aliases: [ performer_credit, performercredit ]
   musicbrainz_performerid:
     aliases: [ txxx:musicbrainz performer id, musicbrainz_performerid, musicbrainz_performer_id, ----:com.apple.itunes:musicbrainz performer id, musicbrainz/performer id ]
     type: pair