Remove path.isfile check

[paivao]

On the line below:

dumpzilla/dumpzilla.py

Line 141 in ec337da

if libnss_path and path.isfile(libnss_path):

you can see that is a check if the libnss_path var is a vaid file, what is always False.

Since CDLL call with handle this correctly, I can assume this is redundant and causes false-negatives.