[K/N] Remove Char.isIdentifierIgnorable() function

As a part of efforts to stabilize K/N stdlib.

kotlin-native/runtime/src/main/cpp/KString.cpp

@@ -339,11 +339,6 @@ KBoolean Kotlin_String_unsafeRangeEquals(KString thiz, KInt thizOffset, KString
   ) == 0;
 }
 
-KBoolean Kotlin_Char_isIdentifierIgnorable(KChar ch) {
-  RuntimeAssert(false, "Kotlin_Char_isIdentifierIgnorable() is not implemented");
-  return false;
-}
-
 KBoolean Kotlin_Char_isISOControl(KChar ch) {
   return (ch <= 0x1F) || (ch >= 0x7F && ch <= 0x9F);
 }