Fix gcc compilation error

[Thoemi09]

gcc complains about the specialized fmt::formatter for nda::MemoryArray types:

#11 22.62 /source/ctseg/c++/triqs_ctseg/logs.hpp:28:43: error: declaration of 'struct fmt::v12::formatter<T>' ambiguates earlier template instantiation for 'struct fmt::v12::formatter<nda::basic_array<double, 1, nda::C_layout, 'V', nda::heap_basic<nda::mem::mallocator<> > >, char, void>' [-fpermissive]
#11 22.62    28 | template <nda::MemoryArray A> struct fmt::formatter<A> : ostream_formatter {};
#11 22.62       |                                           ^~~~~~~~~~~~

Since we are already using fmt::streamed (see 12f816c), I don't think we actually need the formatter. Removing it should be fine and should fix the compilation error.

[Wentzell]

Thank you @Thoemi09 for this fix