Fix incorrect documentation comment left after r321692

TargetRegistryInfo::createMCAsmBackend no longer takes a TheTriple parameter. 
The majory of the TargetRegistryInfo::create* functions have no or very 
limitied per-parameter doc comments, and adding a comment for the 
MCSubtargetInfo, MCRegisterInfo and MCTargetOptions parameters seems like it 
would add no real value beyond reading the function signature. As such, I've 
just deleted the doc comment for TheTriple.

llvm-svn: 321694