Remove now-obsolete destColorspace argument to SkGainmapShader::Make

With https://skia-review.googlesource.com/c/skia/+/986176, the canvas's
working colorspace is used internally (same as prior behavior here), and
this argument is now a no-op -- moving to the new API.

Test: presubmit
Flag: EXEMPT trivial refactor
Bug: N/A
Change-Id: Ibfddbe7689065f34e3e63c3e07e7e9e725211d80