Remove background from textfields to fix errors

Remove background from textfields to fix errors

Fixes the following errors:

ShareeSearchField.qml:53:17: QML QQuickRectangle: The current style does
not support customization of this control (property: "background" item:
QQuickRectangle(0x600001d42140, parent=0x0, geometry=0,0 0x0)).

NCInputTextField.qml:57:17: QML QQuickRectangle: The current style does
not support customization of this control (property: "background" item:
QQuickRectangle(0x600001d42220, parent=0x0, geometry=0,0 0x0)).

Signed-off-by: Claudio Cambra <claudio.cambra@nextcloud.com>