Fix compilation error when using Kokkos's nvcc_wrapper

[ylee88]

Apparently, nvcc with -x cu option parses json::basic_json weirdly, and fails to compile. For example,

mwe.cxx:

template <class T = int>
struct basic_json {
    struct type_error {};
};

using json = basic_json<>;

void func()
{
    try {
    }
    catch (json::basic_json::type_error &) {
    }
}

The code above is a valid C++ and it compiles fine even with nvcc. However, it fails if I pass -x cu:

❯ nvcc -x cu mwe.cxx -c
mwe.cxx: In function ‘void func()’:
mwe.cxx:12:8: error: ‘json {aka basic_json<>}::basic_json’ names the constructor, not the type
   12 |     catch (json::basic_json::type_error &) {
      |        ^~~~
mwe.cxx:12:8: error: and ‘json’ {aka ‘basic_json<>’} has no template constructors
mwe.cxx:12:24: error: expected ‘)’ before ‘<’ token
   12 |     catch (json::basic_json::type_error &) {
      |       ~                ^
      |                        )
mwe.cxx:12:24: error: expected ‘{’ before ‘<’ token
mwe.cxx:12:24: error: expected primary-expression before ‘<’ token
mwe.cxx:12:25: error: expected primary-expression before ‘>’ token
   12 |     catch (json::basic_json::type_error &) {
      |                         ^
mwe.cxx:12:29: error: ‘::type_error’ has not been declared
   12 |     catch (json::basic_json::type_error &) {
      |                             ^~~~~~~~~~
mwe.cxx:12:41: error: expected primary-expression before ‘)’ token
   12 |     catch (json::basic_json::type_error &) {
      |                                         ^

This only matters when openPMD-api is compiled with nvcc_wrapper provided by Kokkos, whose passes -x cu to every source code.

This PR drops the redundant qualifier, as json is already an alias for basic_json<>, and make it compilable even with Kokkos's nvcc_wrapper.

[franzpoeschel]

Thank you!