collapse parametric_{r,g,b} into parametric, remove _a and gamma_dst

parametric_a and gamma_dst were unused outside of unit tests.

In all other cases, we always use parametric_{r,g,b} together
and always pass them the same argument.  So we can collapse
them into a single stage like gamma and to/from_srgb.

Change-Id: I08cea896c7744f97b4f4bf9e029f5d643e45e177
Reviewed-on: https://skia-review.googlesource.com/140576
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Brian Osman <brianosman@google.com>

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