Remove long obsolete feature checking API

* src/global.h (enum Feature, enum Feature2, hasFeature): Remove.
* src/context.cpp (supported_features, supported_features2, hasFeature):
Remove.

gpgme has all features that could be checked for since version 1.7.0
(released almost 9 years ago) and gpgmepp requires gpgme 2.0 anyway.
The times when gpgmepp could be built against an older version of gpgme
than itself are long gone.