Revert "[gdb/symtab] Fix infinite recursion in dwarf2_cu::get_builder()"

This reverts commit 4cf88725.

It causes the following regression:
...
$ cat shadow.cc
namespace A {}

int
main()
{
  using namespace A;
  return 0;
}
$ g++-10 -g shadow.cc -flto -o shadow
$ ./gdb -q -batch ./shadow  -ex "b main"
Aborted (core dumped)
...