Remove IECoreHoudini

This will continue life as an internal Image Engine project.

Author: johnhaddon

Changes

@@ -8,6 +8,11 @@ Fixes
   - Fixed writing of shutter values from cameras without a `shutter` parameter. The `shutter:open` and `shutter:close` attributes are now omitted instead of being written with Cortex's default -0.5, 0.5 shutter values.
   - Fixed reading of shutter values from cameras without `shutter:open` and `shutter:close` attributes. The `shutter` parameter is now omitted instead of being created with USD's default 0, 0 shutter values.
 
+Breaking Changes
+----------------
+
+- IECoreHoudini : Removed.
+
 10.5.x.x (relative to 10.5.15.0)
 ========