core: Remove experimental feature GPGME_EXPORT_MODE_NOUID.

* src/export.c (export_start): Remove GPGME_EXPORT_MODE_NOUID check.
* src/engine-gpg.c (export_common): Ditto.

Note that this constant was never announed and used only for internal
tests. It made it into some release only by accident.

The constants is kept in gpgme.h to not break any code but it is
non-fucntionals. We do not consider this an ABI break.

• GnuPG-bug-id: T5284