Fix Doxygen warnings, remove some unused code

[abitmore]

PR for #2213.

Bumped FC with bitshares/bitshares-fc#223 .

NOTE:
Item(s) unable to fix so far:

• warning: documented symbol `graphene::chain::decltype' was not declared or defined on several files
  • see https://sourceforge.net/p/doxygen/mailman/message/36304025/ - Doxygen doesn’t support decltype() yet
• warning: no uniquely matching class member found for (some methods in some template classes), E.G.

libraries/fc/include/fc/fwd_impl.hpp:128: warning: no uniquely matching class member found for
  template < T, S, A >
  T & fc::fwd< T, S, A >::operator=(fc::fwd< T, S, A > &&u)
Possible candidates:
  'template < U >
  T & fc::fwd< T, S, Align >::operator=(U &&u)' at line 31 of file /home/ubuntu/bitshares-core-dev/libraries/fc/include/fc/fwd.hpp
  T & fc::fwd< T, S, Align >::operator=(fwd &&u)' at line 33 of file /home/ubuntu/bitshares-core-dev/libraries/fc/include/fc/fwd.hpp
  T & fc::fwd< T, S, Align >::operator=(const fwd &u)' at line 34 of file /home/ubuntu/bitshares-core-dev/libraries/fc/include/fc/fwd.hpp

• libraries/fc/include/fc/thread/parallel.hpp:91: warning: argument 'desc' of command @param is not found in the argument list of fc::do_parallel(Functor &&f, const char *desc FC_TASK_NAME_DEFAULT_ARG) ->-> fc::future< decltype(f())> (and some other files), and parameter 'FC_TASK_NAME_DEFAULT_ARG' is "not documented"
  • Doxygen thinks FC_TASK_NAME_DEFAULT_ARG is the argument
• README.md:6: warning: Unexpected html tag <img> found within <a href=...> context
  • The section will be removed from the generated HTML file anyway