Remove assertion that library blend files always exist

Remove an assertion that would cause BAT to raise and exception and halt
when a library blend file is missing. This assertion was fine for aiding
the coding of the library, but now gets in the way of tracing dependencies
of partially checked-out projects.

blender_asset_tracer/trace/file2blocks.py

@@ -125,7 +125,6 @@ class BlockIterator:
         for lib_bpath, idblocks in blocks_per_lib.items():
             lib_path = bpathlib.make_absolute(lib_bpath.to_path())
 
-            assert lib_path.exists()
             if not lib_path.exists():
                 log.warning('Library %s does not exist', lib_path)
                 continue