ign -> gz : Remove redundant namespace references

[methylDragon]

Part of: https://github.com/gazebo-tooling/release-tools/issues/784

[codecov[bot]]

Codecov Report

Merging #1635 (2f3afcb) into ign-gazebo3 (1cf75f3) will increase coverage by 0.03%. The diff coverage is 73.52%.

@@               Coverage Diff               @@
##           ign-gazebo3    #1635      +/-   ##
===============================================
+ Coverage        77.94%   77.98%   +0.03%     
===============================================
  Files              255      255              
  Lines            15041    15041              
===============================================
+ Hits             11724    11730       +6     
+ Misses            3317     3311       -6     

Impacted Files	Coverage Δ
include/ignition/gazebo/EntityComponentManager.hh	100.00% <ø> (ø)
...e/ignition/gazebo/detail/EntityComponentManager.hh	95.20% <ø> (ø)
src/gui/GuiFileHandler.cc	0.00% <0.00%> (ø)
src/gui/TmpIface.cc	4.76% <0.00%> (ø)
src/systems/battery_plugin/LinearBatteryPlugin.cc	42.08% <ø> (ø)
src/systems/detachable_joint/DetachableJoint.cc	70.42% <ø> (ø)
src/systems/imu/Imu.cc	71.42% <ø> (ø)
...int_position_controller/JointPositionController.cc	73.07% <ø> (ø)
src/systems/magnetometer/Magnetometer.cc	70.78% <ø> (ø)
.../multicopter_control/MulticopterVelocityControl.cc	72.39% <ø> (ø)
... and 44 more

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[methylDragon]

Seems this is good to go too!