[EMSUSD-2154] mayaUsd_ProxyTargetsSessionLayerOnOpen doesn't seem to work

**Describe the bug**
The mayaUsd_ProxyTargetsSessionLayerOnOpen option var does not appear to work for us with mayaUsdProxyShapes anymore. We should expect that the session layer is always set as the edit target, regardless of what is set in Maya when the file is saved.

**Steps to reproduce**
Steps to reproduce the behaviour:
1. Set the mayaUsd_ProxyTargetsSessionLayerOnOpen option var to 1.
2. Create a mayaUsdProxyShape pointing to a file.
3. Save a Maya file with the proxy shape included.
4. Close the Maya file and reopen the one saved in step 4.
5. Look at the Layer Editor and you will see that the session layer is not targeted despite the option var.

**Expected behavior**
The session layer should be targeted regardless of the edit target saved into the file.

**Specs (if applicable):**
- OS & version : Windows 10
- Compiler & version : precompiled
- Maya version : 2022.5, 2024
- Maya USD : 0.30.0