Bug 1656611 - Remove objdir support from virtualenv_packages.txt handling r=mhentges,froydnj

I noticed that the `objdir:build` entry in `build/virtualenv_packages.txt` entry was apparently unused. This originates from bug 841713, seven years ago, when the `objdir` handling was introduced. Today, this doesn't appear to be serving a purpose. There is no Python library in my `$objdir/build` directory; nor can I see anything in `build/moz.build` or any related files suggesting one could ever appear. I can only assume this feature has outlived its usefulness, so delete it and the relevant in-tree support.

This necessitates slightly changing the signature and implementation of the `activate_pipenv()` method; also update all callers.

Differential Revision: https://phabricator.services.mozilla.com/D85635

python/mach_commands.py

@@ -253,7 +253,8 @@ class MachCommands(MachCommandBase):
         python = python or default_manager.python_path
         py3_root = default_manager.virtualenv_root + '_py3'
 
-        self.activate_pipenv(pipfile=None, populate=True, python=python)
+        self.activate_pipenv(os.path.dirname(default_manager.virtualenv_root),
+                             pipfile=None, populate=True, python=python)
 
         # The current process might be running under Python 2 and the Python 3
         # virtualenv will not be set up by mach bootstrap. To avoid problems in tests
@@ -263,7 +264,6 @@ class MachCommands(MachCommandBase):
 
         py3_manager = VirtualenvManager(
             default_manager.topsrcdir,
-            default_manager.topobjdir,
             py3_root,
             default_manager.log_handle,
             default_manager.manifest_path,