[ProfileData] Remove getHostEndianness (NFC)

With the recent redefinition of llvm::endianness::native, it is equal
to either llvm::endianness::big or llvm::endianness::little depending
on the host endianness.  Since getHostEndianness just returns
llvm::endianness::native, this patch removes the function and
"constant propagates" llvm::endianness:native.