Simplify nginx.conf

Update nginx.conf based on the changes in https://github.com/fossar/selfoss/wiki/Example-step-by-step-installation-using-nginx-and-SQLite/_compare/21348062e1d7ecd791c4dedb2d13c7505902e95b...27774f3286879b2ca2c3014d5ca89c99cd58078c#diff-e2d489d2b628bc165b907b5aa30ad4b1e05aeaa6c603f0423abeb42ff2bfb65bR101-R102