Use _force_rmtree instead of custom code.

Description

Use _force_rmtree instead of the custom code in element.py to ensure removal of a directory which may contain nodes without read permission.

The functions are not quite identical; the existing code in _stage_sources_at uses onerror as a potential performance improvement, but testing hasn't shown it to actually improve performance. This is discussed in issue #735 (closed).

Proposed changes

Changes proposed in this merge request:

• Delete the custom code in _stage_sources_at and use _force_rmtree instead.

This merge request, when approved, will close: #735 (closed)

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