[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

libraries/stdlib/js/src/kotlin/text/stringsCode.kt

@@ -54,13 +54,6 @@ public fun String.matches(regex: String): Boolean {
     return result != null && result.size != 0
 }
 
-/**
- * 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 `true` if this string is equal to [other], optionally ignoring character case.
  *