[stdlib] Use more idiomatic and performant implementation of isBlank

Also replace expect-actual with a common function because its implementation is no longer different in different platforms.

KT-65590

kotlin-native/runtime/src/main/kotlin/kotlin/text/Strings.kt

@@ -114,13 +114,6 @@ public actual fun String.replaceFirst(oldValue: String, newValue: String, ignore
     return if (index < 0) this else this.replaceRange(index, index + oldValue.length, newValue)
 }
 
-/**
- * Returns `true` if this string is empty or consists solely of whitespace characters.
- *
- * @sample samples.text.Strings.stringIsBlank
- */
-public actual fun CharSequence.isBlank(): Boolean = length == 0 || indices.all { this[it].isWhitespace() }
-
 /**
  * Returns the substring of this string starting at the [startIndex] and ending right before the [endIndex].
  *