lint: remove unused structs and functions (unused)

Signed-off-by: Moritz Poldrack <moritz@poldrack.dev>
Acked-by: Robin Jarry <robin@jarry.cc>

widgets/compose.go

@@ -995,13 +995,6 @@ func (he *headerEditor) storeValue() {
 	}
 }
 
-// setValue overwrites the current value of the header editor and flushes it
-// to the underlying header
-func (he *headerEditor) setValue(val string) {
-	he.input.Set(val)
-	he.storeValue()
-}
-
 func (he *headerEditor) Draw(ctx *ui.Context) {
 	name := textproto.CanonicalMIMEHeaderKey(he.name)
 	// Extra character to put a blank cell between the header and the input