Remove duplicate prop definition

Summary:
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- This is most likely a merge issue?
- hasTVPreferredFocus was duplicated in the Flow types definition for the Button component, but the PropTypes are fine

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Removing a duplicated prop can't really throw a Flow error. Anyway, I removed the duplicated prop and successfully ran `$ yarn flow`

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[GENERAL] [ENHANCEMENT] [Button] - Remove duplicate prop definition

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    CATEGORY
  [----------]      TYPE
  [ CLI      ] [-------------]    LOCATION
  [ DOCS     ] [ BREAKING    ] [-------------]
  [ GENERAL  ] [ BUGFIX      ] [ {Component} ]
  [ INTERNAL ] [ ENHANCEMENT ] [ {Filename}  ]
  [ IOS      ] [ FEATURE     ] [ {Directory} ]   |-----------|
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 EXAMPLES:

 [IOS] [BREAKING] [FlatList] - Change a thing that breaks other things
 [ANDROID] [BUGFIX] [TextInput] - Did a thing to TextInput
 [CLI] [FEATURE] [local-cli/info/info.js] - CLI easier to do things with
 [DOCS] [BUGFIX] [GettingStarted.md] - Accidentally a thing/word
 [GENERAL] [ENHANCEMENT] [Yoga] - Added new yoga thing/position
 [INTERNAL] [FEATURE] [./scripts] - Added thing to script that nobody will see
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Closes https://github.com/facebook/react-native/pull/18845

Differential Revision: D7627621

Pulled By: mdvacca

fbshipit-source-id: 924522641a334212f5e2c1310d81da1e321f19b1

Libraries/Components/Button.js

@@ -57,7 +57,6 @@ class Button extends React.Component<{
   accessibilityLabel?: ?string,
   disabled?: ?boolean,
   testID?: ?string,
-  hasTVPreferredFocus?: ?boolean,
 }> {
   static propTypes = {
     /**