CDRIVER-1544: Remove _Decimal128 functiona

These are pretty useless anyway, only work on platforms with
BID formatted _Decimal128 type, in which case the user can do the cast
themselfs anyway.
It also causes problems with C++, where if mongoc is compiled and
installed normally, but then the headers consumed by c++ compiler, our
_Decimal128 guards are broken as they don't realize C++ doesn't have
these, even thoug the original c compiler did.
Same problem occurrs when compiling and installing mongoc using
compatible compiler and platform, but then consuming the header with
say clang, which doesn't support the type either.

All in all, the functions create more troubles then they are worth.
We should add bson_decimal128_to_dpd() instead in the future, which
would be more helpful