Popped keys instead of deleting them.

If a residue has chi corrections and both atoms in the dictionary are
absent, the code attempts to delete non-existent keys.  The code has
been changed to pop the keys, which will prevent the KeyError from
being thrown.

Refs #17229
Merge branch 'bug/17229' into develop

After discussion with Dale and Ken, this change can be merged into develop.

Refs #17229