Last step of proxy building caused crash due to thread race contition
when acessing ed->seqbase.
Looking at code, it seems that calling IMB_close_anim_proxies on
original strips is unnecessary so this code was removed to resolve this
issue.
Locking seqbase data may be possible, but it's not very practical as
many functions access this data on demand which can easily cause
program to freeze.
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D14210
4491c62260