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/feature/api/feature_api.dox

@@ -1,4 +1,2 @@
-/**
 @dir /feature/api
 @brief feature/api: In-process interface to starting/stopping Tor.
-**/