[JS IR] [runtime] Remove valueOf method from Long

This method was used for coercing `Long` values to JavaScript
`number`. However, it caused issues when `Long` values were
concatenated to a string (see KT-8666, KT-26706).

libraries/stdlib/js-ir/runtime/long.kt

@@ -279,13 +279,9 @@ public class Long internal constructor(
     public override fun toFloat(): Float = toDouble().toFloat()
     public override fun toDouble(): Double = toNumber()
 
-    // This method is used by `toString()`
-    @JsName("valueOf")
-    internal fun valueOf() = toDouble()
-
     override fun equals(other: Any?): Boolean = other is Long && equalsLong(other)
 
     override fun hashCode(): Int = hashCode(this)
 
     override fun toString(): String = this.toStringImpl(radix = 10)
-}
+}