refactor(upgrade): Drop the superseded upgrade preamble card

The hero card took over both call sites (FOSS pitch, GPLAY acquisition), so
the standalone preamble card had no callers left.

Fixes review finding F1.

app/src/main/java/eu/darken/capod/common/upgrade/ui/UpgradeContent.kt

@@ -385,26 +385,6 @@ private fun UpgradeHeroCardTintedPreview() {
     }
 }
 
-@Composable
-internal fun UpgradePreambleCard(
-    text: String,
-    modifier: Modifier = Modifier,
-    colors: CardColors = CardDefaults.elevatedCardColors(),
-) {
-    ElevatedCard(
-        modifier = modifier.fillMaxWidth(),
-        colors = colors,
-    ) {
-        Text(
-            text = text,
-            style = MaterialTheme.typography.bodyMedium,
-            modifier = Modifier
-                .fillMaxWidth()
-                .padding(16.dp),
-        )
-    }
-}
-
 @Composable
 internal fun UpgradeSectionCard(
     title: String,