Spinner - remove legacy variant

[mcoker]

Remove support for the non-SVG spinner.

re: this core issue, though this shouldn't need to wait for core - patternfly/patternfly#3827

[nicolethoen]

If someone was using the non-svg, now by default they would be using the svg.
we can remove the isSvg prop. and the code mod can remove the prop. There was a code mod already updating the isSvg prop, so we can update that existing code mod rather than create a new one