Remove Pittsburgh PA source (redundant with Allegheny County, PA - same ...

...source link)

This PR removes the Pittsburgh, PA source ( https://github.com/openaddresses/openaddresses/blob/master/sources/us-pa-pittsbugh.json ) since there already exists a more recent (and properly-spelled) file for the entirety of Allegheny County ( https://github.com/openaddresses/openaddresses/blob/master/sources/us-pa-allegheny.json ).

For clarity, the two files reference the same data source, just different dates.