Drop remainder of files in libgnucash/doc

Relevant diagrams have been added in the wiki as png files.
The html files with financial calculations are copyrighted
by someone outside of the gnucash project. I'd rather not
add that info to the wiki. Similar information should be
easy to find on the internet.

Author: gjanssens

libgnucash/CMakeLists.txt

@@ -4,7 +4,6 @@
 add_subdirectory (app-utils)
 add_subdirectory (backend)
 add_subdirectory (core-utils)
-add_subdirectory (doc)
 add_subdirectory (engine)
 add_subdirectory (gnc-module)
 add_subdirectory (quotes)