remove REBUILD command from Makefile
remove REBUILD command from Makefile

Previously, it was required to touch the release constant files in order
for them to be built properly in the event that you were changing
release constants. This is no longer necessary in more recent versions
of go, so the REBUILD command has been removed.

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