ign -> gz : Remove redundant namespace references

[methylDragon]

Part of: gazebo-tooling/release-tools#784

[codecov]

Codecov Report

Merging #1105 (7e3ccfe) into sdf9 (3e76d92) will not change coverage.
The diff coverage is 86.01%.

@@           Coverage Diff           @@
##             sdf9    #1105   +/-   ##
=======================================
  Coverage   87.59%   87.59%           
=======================================
  Files          64       64           
  Lines       10061    10061           
=======================================
  Hits         8813     8813           
  Misses       1248     1248           

Impacted Files	Coverage Δ
src/Sensor.cc	77.70% <30.00%> (ø)
src/Geometry.cc	87.59% <42.85%> (ø)
src/Model.cc	83.70% <50.00%> (ø)
src/Camera.cc	81.16% <70.00%> (ø)
src/World.cc	93.45% <77.77%> (ø)
src/Pbr.cc	98.49% <83.33%> (ø)
src/Console.cc	87.71% <85.71%> (ø)
src/Actor.cc	83.33% <100.00%> (ø)
src/AirPressure.cc	100.00% <100.00%> (ø)
src/Altimeter.cc	100.00% <100.00%> (ø)
... and 32 more

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[scpeters]

I think the namespace reduction plan in gazebo-tooling/release-tools#784 is useful for eliminating ignition:: namespaces, but for this library it is just removing sdf:: in lots of places, which looks like noise to me. I would rather just decline this.

[chapulina]

This PR isn't strictly helpful for the ignition migration because this repository doesn't release any code under that namespace, so we shouldn't be using namespace ignition here.

But I think removing those redundant namespaces doesn't hurt, makes some liner shorter, and the code a bit cleaner. It may just introduce some confusion for functions that aren't clearly from this library, like sdf::split

[chapulina]

I just saw @scpeters 's comment, I'm fine declining this PR if that's preferred.

[methylDragon]

I have no preferences, but if needed, I can exclude stuff like split from the cleanup, it's not too difficult for me to do (like a 5-10 min job)

[chapulina]

@methylDragon , I'll just close this one for expediency. We don't need to put more time into it. Let's focus on ignition:: and ignition::gazebo.