Make endpoint for deleting file contracts a POST
Make endpoint for deleting file contracts a POST

instead of a GET request. This matches the API for deleting a file entry
from the renter, which is also a POST request.

MrStrong/Sia,ChrisSchinnerl/Sia,mnsl/Sia,rivine/rivine,ChrisSchinnerl/Sia,rivine/rivine,mharkus/Sia,mtlynch/Sia,ericflo/Sia,johnathanhowell/Sia,mnsl/Sia,NebulousLabs/Sia,MrStrong/Sia,mharkus/Sia,ericflo/Sia,mtlynch/Sia,NebulousLabs/Sia