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/thread/lib_thread.dox

@@ -1,4 +1,3 @@
-/**
 @dir /lib/thread
 @brief lib/thread: Mid-level threading.
 
@@ -6,4 +5,3 @@ This module contains compatibility and convenience code for multithreading,
 except for low-level locks (which are in \refdir{lib/lock} and
 workqueue/threadpool code (which belongs in \refdir{lib/evloop}.)
 
-**/

src/lib/thread/threading.dox

@@ -1,4 +1,3 @@
-/**
 
 @page threading Threading in Tor
 
@@ -25,4 +24,3 @@ Try to minimize sharing between threads: it is usually best to simply
 make the worker "own" all the data it needs while the work is in
 progress, and to give up ownership when it's complete.
 
-**/