EXT-5629 FIXED Made sensitivity of +/- zoom buttons from camera floater consistent with viewer 1.23

- Changed actions happening when minus or plus are held- now onSliderValueChanged() is not used for zoom via buttons. Zooming in and out now happens as it did
in 1.23 in LLJoystickCameraZoom::onHeldDown(), but only "Zoom in slow" and Zoom out slow" were borrowed from there, because fast versions are not needed- they do not happen in 1.23.

- Removed LLJoystickCameraZoom class, because it's not used anywhere. If need for it ever rises, it can be easily restored.

Reviewed by Vadim Savchuk at https://codereview.productengine.com/secondlife/r/530/

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branch : product-engine