Doxygen: remove /** and **/ from all .dox files

This is an automatically generated commit, made with:

find src -name '*.dox' | \
   xargs  perl -i -ne 'print unless (m#^\s*/?\*\*/?\s*$#);'

src/lib/encoding/lib_encoding.dox

@@ -1,8 +1,6 @@
-/**
 @dir /lib/encoding
 @brief lib/encoding: Encoding data in various forms, types, and transformations
 
 Here we have time formats (timefmt.c), quoted strings (qstring.c), C strings
 (string.c) base-16/32/64 (binascii.c), and more.
 
-**/