demangle: remove an old TODO comment

Summary:
Both the autoconf and cmake build scripts check for
`cplus_demangle_v3_callback()` and define
`FOLLY_HAVE_CPLUS_DEMANGLE_V3_CALLBACK` appropriately.

Reviewed By: meyering

Differential Revision: D6745365

fbshipit-source-id: 3020593c60a21568781b835490b8d64913366cb9

folly/Demangle.cpp

@@ -27,8 +27,6 @@
 
 // From libiberty
 //
-// TODO(tudorb): Detect this with autoconf for the open-source version.
-//
 // __attribute__((__weak__)) doesn't work, because cplus_demangle_v3_callback
 // is exported by an object file in libiberty.a, and the ELF spec says
 // "The link editor does not extract archive members to resolve undefined weak