Remove spurious "typedef" in type declaration when using "using"

[tristan0x]

Remove the spurious "typedef " on right hand side added
by Doxygen observed when type is declared inside a namespace
See examples/specific/using_in_ns

//
// When declaring a type using a "using" directive inside a namespace,
// Doxygen adds a spurious "typedef" in the corresponding XML definition
//
// $ doxygen --version
// 1.8.11
//

namespace foo {
using foo_int = int; // <definition>using foo::foo_int = typedef int</definition>
}

using global_int = int; // <definition>using global_int =  int</definition>

It fixes warnings issued by Sphinx when processing the docstring:

/path/to/using_in_ns.rst:10: WARNING: Error in type declaration.
If typedef-like declaration:
  Type must be either just a name or a typedef-like declaration.
  If just a name:
    Invalid definition: Expected end of definition. [error at 19]
      foo::foo_int = typedef int

• The extra "typedef" has been observed in XML written by Doxygen 1.8.11
• The warning above has been observed with Sphinx 1.8.5

[vermeeren]

Seems good to me, thanks for the patch!