Remove autoclear option from squashfs mount

[uvNikita]

Unless I'm missing something, it seems like it's not supported in the new kernels.

Without this patch I'm getting "Unknown parameter autoclear" in dmesg when attempting to mount squashfs image.
I think old kernels might have just ignored unsupported options.

[coveralls]

Coverage remained the same at 78.399% when pulling cb7c321 on uvNikita:squashfs/autoclear into 771dcc4 on NERSC:master.

[scanon]

@dmjacobsen Can you look at this?

[scanon]

I've run into this as well. I'm still discussing with @dmjacobsen the ramifications of disabling it. But the newer kernels simply do not recognize this option.

[scanon]

We have confirmed that removing the autoclear is safe. @dmjacobsen was concerned about. We will work on a different fix that will somehow (TBD) determine if the flag should be used or not. Leaving off the autoclear on older kernels will result in orphaned loopback mounts.