test: Remove annotation for now-fixed FreeBSD issue pr15302

The failure to demangle 'anonymous namespace' on FreeBSD is fixed (twice)

- the failure in FreeBSD's in-tree __cxa_demangle has been addressed

- FreeBSD now uses the copy of the demangler built into lldb, due to other
  remaining limitations in the in-tree __cxa_demangle

llvm.org/pr15302