Change cover memory key (#7276)

Use different key for custom cover and add last modified time for updating
cover without clearing the whole memory cache

app/src/main/java/eu/kanade/tachiyomi/data/cache/CoverCache.kt

@@ -1,7 +1,6 @@
 package eu.kanade.tachiyomi.data.cache
 
 import android.content.Context
-import coil.imageLoader
 import eu.kanade.tachiyomi.data.database.models.Manga
 import eu.kanade.tachiyomi.util.storage.DiskUtil
 import java.io.File
@@ -100,13 +99,6 @@ class CoverCache(private val context: Context) {
         }
     }
 
-    /**
-     * Clear coil's memory cache.
-     */
-    fun clearMemoryCache() {
-        context.imageLoader.memoryCache?.clear()
-    }
-
     private fun getCacheDir(dir: String): File {
         return context.getExternalFilesDir(dir)
             ?: File(context.filesDir, dir).also { it.mkdirs() }