Merge #612: refactor: Drop unused QFrames in SendCoinsEntry

7ab72b9 qt: Fix `BitcoinAmountField`'s base widget (Hennadii Stepanov)
3262542 qt, refactor: Fix `sendcoinsentry.ui` indentation (Hennadii Stepanov)
f3c7603 qt, refactor: Convert `SendCoinsEntry` to a sub-`QWidget` (Hennadii Stepanov)
6420fb2 qt, refactor: Drop unused `QFrame`s in `SendCoinsEntry` (Hennadii Stepanov)

Pull request description:

  The `SendCoins_UnauthenticatedPaymentRequest` and `SendCoins_AuthenticatedPaymentRequest` sub-`QFrame`'s of the `SendCoinsEntry` widget have been unused since bitcoin/bitcoin#17165.

  Removed all dead code. The resulted `SendCoinsEntry` widget has been simplified.

ACKs for top commit:
  w0xlt:
    Tested ACK 7ab72b9
  shaavan:
    reACK 7ab72b9

Tree-SHA512: a46db90d60fae584b52cc7edae910c295351cb3627e04d225708c50c04f7fdd81d2755e055115612a12a3c841e78c31bdcd57bed9feb1d3909f7a2f6e76bd356

Author: hebasto