clean up old idea about nullptr/sRGB
clean up old idea about nullptr/sRGB

I don't see us ever really saying dstCS == nullptr --> dstCS = sRGB.
It's too handy a feature to have a legacy dst.

Change-Id: I20576962eeb5a2ce7d0da09ffe1bbba860dad086
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/297418
Auto-Submit: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia