Delete JSONCPP_DEPRECATED, use [[deprecated]] instead

[baylesj]

Is your feature request related to a problem? Please describe.
We have a hacky, compiler specific macro due to being pre C++11. This should be replaced with a standard language feature.

Describe the solution you'd like
Deletion of JSONCPP_DEPRECATED in favor of C++11's language standard deprecated

[dota17]

Actually, [[deprecated]] is a feature in C++14, I made a typo in #978 's description:

Language Conformance: Deletion of JSONCPP_DEPRECATED in favor of C++11C++14's language standard [[deprecated]]
Tips： #978 PR is wrote for #975 [Migrate to C++14].

Maybe there will be some compatibility problems like issue #990.