Do not poke into the protected _Addr member of std::error_category

[pdimov]

... as this creates problems for the MS STL (microsoft/STL#3176).

Since a nonzero id is only passed when BOOST_SYSTEM_AVOID_STD_GENERIC_CATEGORY or BOOST_SYSTEM_AVOID_STD_SYSTEM_CATEGORY is defined, and since id is only stored into _Addr when the MS STL is 14x, but none of these macros are defined in this case, this code should be dead and it should be possible to remove it.