doxygen: Avoid segfault.

After an upgrade to Ubuntu-16.04, the new doxygen version 1.8.11
started to segfault. It turned out that this can be circumvented
by disabling USE_HTAGS so that doxygen will use its built-in source
browser instead of the htags tool of GNU global.

However, the html which is generated by the built-in source browser
the source code is unreadable because leading tab characters are not
shown, so indentation is messed up.

This patch deactivates the source browser for now, getting rid of
all source code in the generated output.