Merge bitcoin-core/gui#85: Remove unused "What's This" button in dialogs on Windows OS

ac7ccd67d7 scripted-diff: Remove unused "What's This" button in dialogs on Windows (Hennadii Stepanov)
b6951483ec qt: Add flags to prevent a "What's This" button on Windows OS (Hennadii Stepanov)

Pull request description:

  Fix #74.

  From [Qt docs](https://doc.qt.io/qt-5/qdialog.html#QDialog):
  > The widget flags _f_ are passed on to the `QWidget` constructor. If, for example, you don't want a **What's This** button in the title bar of the dialog, pass `Qt::WindowTitleHint | Qt::WindowSystemMenuHint` in _f_.

  Screenshot on Windows 10 (2004):
  - master (3ba25e3bdd)
  ![Screenshot from 2020-09-07 16-55-42](https://user-images.githubusercontent.com/32963518/92402384-20dc6a00-f138-11ea-9dcb-3e0f6373ff22.png)

  - this PR (e322fe7e19ac504272d14b9b4f9b28b13df888ed)
  ![Screenshot from 2020-09-07 18-31-16](https://user-images.githubusercontent.com/32963518/92402509-5aad7080-f138-11ea-8b63-9bbbf8b9b9e1.png)

ACKs for top commit:
  Bosch-0:
    tACK ac7ccd67d7 Tested on Windows 10.0.18363 Build 18363.
  promag:
    Code review ACK ac7ccd67d7 but with some suggestions.
  jonasschnelli:
    utACK ac7ccd67d7

Tree-SHA512: f6750a17b7203106cb4db5870becba1cef6a505d4edcc710ba131338bd3aae051510627e62c9bcb8345a7f497c614709e11aeb8f6ae3ea85967bbce2a8c69e64

src/qt/sendcoinsdialog.cpp

@@ -55,7 +55,7 @@ int getIndexForConfTarget(int target) {
 }
 
 SendCoinsDialog::SendCoinsDialog(const PlatformStyle *_platformStyle, QWidget *parent) :
-    QDialog(parent),
+    QDialog(parent, GUIUtil::dialog_flags),
     ui(new Ui::SendCoinsDialog),
     clientModel(nullptr),
     model(nullptr),