Remove long unsupported "support" for assuan 1
Assuan 2 is a hard requirement:
- Remove all support for NOT ASSUAN2_FOUND from CMakeLists.txt files
- Remove obsolete configure checks and the corresponding defines HAVE_ASSUAN_FD_T, HAVE_ASSUAN_INQUIRE_EXT, HAVE_NEW_STYLE_ASSUAN_INQUIRE_EXT, and HAVE_ASSUAN_SOCK_GET_NONCE.
- Remove obsolete define HAVE_ASSUAN2.
- GnuPG-bug-id: T6310