shelve: fix a missing variable in the exception handler for delete
Caught by pytype. I haven't paid much attention to the progress of this
extension, but I *think* this was the intent.
Differential Revision: https://phab.mercurial-scm.org/D7457
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL