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/string/lib_string.dox

@@ -1,4 +1,3 @@
-/**
 @dir /lib/string
 @brief lib/string: Low-level string manipulation.
 
@@ -12,4 +11,3 @@ in the rest of the codebase.
 
 Any string function high-level enough to need logging belongs in a
 higher-level module.
-**/

src/lib/string/strings.dox

@@ -1,4 +1,3 @@
-/**
 
 @page strings String processing in Tor