Remove compatible system code for iOS8 and before (#23656)

Summary:
We already only support `iOS9+`, so we can remove all compatible codes now.

[iOS] [Fixed] - Remove compatible system code for iOS8 and before
Pull Request resolved: https://github.com/facebook/react-native/pull/23656

Differential Revision: D14224986

Pulled By: hramos

fbshipit-source-id: cac9ffe6788dd3eaf4f4f5f2b219f325ba78e85f

Libraries/Text/TextInput/Multiline/RCTUITextView.m

@@ -222,37 +222,12 @@ static UIColor *defaultPlaceholderColor()
 - (CGSize)sizeThatFits:(CGSize)size
 {
   // Returned fitting size depends on text size and placeholder size.
-  CGSize textSize = [self fixedSizeThatFits:size];
+  CGSize textSize = [super sizeThatFits:size];
   CGSize placeholderSize = self.placeholderSize;
   // Returning size DOES contain `textContainerInset` (aka `padding`).
   return CGSizeMake(MAX(textSize.width, placeholderSize.width), MAX(textSize.height, placeholderSize.height));
 }
 
-- (CGSize)fixedSizeThatFits:(CGSize)size
-{
-  // UITextView on iOS 8 has a bug that automatically scrolls to the top
-  // when calling `sizeThatFits:`. Use a copy so that self is not screwed up.
-  static BOOL useCustomImplementation = NO;
-  static dispatch_once_t onceToken;
-  dispatch_once(&onceToken, ^{
-    useCustomImplementation = ![[NSProcessInfo processInfo] isOperatingSystemAtLeastVersion:(NSOperatingSystemVersion){9,0,0}];
-  });
-
-  if (!useCustomImplementation) {
-    return [super sizeThatFits:size];
-  }
-
-  if (!_detachedTextView) {
-    _detachedTextView = [UITextView new];
-  }
-
-  _detachedTextView.attributedText = self.attributedText;
-  _detachedTextView.font = self.font;
-  _detachedTextView.textContainerInset = self.textContainerInset;
-
-  return [_detachedTextView sizeThatFits:size];
-}
-
 #pragma mark - Context Menu
 
 - (BOOL)canPerformAction:(SEL)action withSender:(id)sender