Remove Help command

* src/gpgex.cc (getCaptionForId, GetCommandString),
(get_lang_name): Remove help handling.
* src/gpgex.cc (start_help): Remove help.

The help has been full of FIXMEs forever and I
do not really see what could be documented about
GpgEX in user Help. Help can be found in Kleopatra.

• GnuPG-Bug-Id: T6317