Remove HQ2X and the `Image.expand_2x_hq2x()` method

As of Godot 3.0, HQ2X is no longer used to upscale the editor theme
and icons on hiDPI displays, which limited its effective uses.
HQ2X was also used to upscale the project theme when the "Use Hidpi"
project setting was enabled, but results were often less than ideal.
The new StyleBoxFlat and SVG support also make HQ2X less important
to have as a core feature.

This decreases binary sizes slightly (-150 KB on most platforms,
-212 KB on WebAssembly release).

This partially addresses #12419.

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