[X86] Delete dead code in fcopysign lowering. NFC.

r32900 introduced custom lowering for fcopysign, with two checks to
change the magnitude value's type if it's larger/smaller than the sign
value's type.  r32932 replaced that code for the smaller case.
r43205 did the same for the larger case, but left the old code, now dead.

llvm-svn: 223415