gh-150886: Remove unused importlib._bootstrap._object_name (#150884)

gh-150886: Remove unused importlib._bootstrap._object_name (#150884)

Introduced in GH-23469 (bpo-26131, "Deprecate usage of load_module()")
to render an object's qualified name inside the load_module()
deprecation warnings.

Orphaned by gh-142205 (GH-97850, "Remove all uses and definitions of
load_module() from importlib"), which deleted the warning-building call
sites f"{_object_name(spec.loader)}.exec_module() not found; ..." and
left the helper with no caller.

A word-boundary search across Lib, Modules, Python, Objects and Include
finds zero references outside its own definition, and a GitHub code
search finds no downstream importers. The frozen importlib was
regenerated; importlib._bootstrap._object_name no longer exists at
runtime and the full test_importlib suite passes.