Remove engine-gpgme

* src/engine-gpgme.c, src/engine-gpgme.h: Remove historic code.
* src/engine.c: Assume use_assuan and remove engine-gpgme calls.
* src/Makefile.am: Update accordingly.

The fallback to the gpgme-engine without assuan was unmaintained
and did not provide all the features anyway. It was mostly there
for historic reasons as this predates assuan.