[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/jvm/src/kotlin/text/StringsJVM.kt

@@ -614,13 +614,6 @@ public actual fun CharSequence?.contentEquals(other: CharSequence?, ignoreCase:
 @kotlin.internal.InlineOnly
 public inline fun String.intern(): String = (this as java.lang.String).intern()
 
-/**
- * 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 index within this string that is offset from the given [index] by [codePointOffset] code points.
  */