Remove unnecessary FIXIT section in grfmt_tiff.cpp

[vrabaud]

No int64/uint64 is used in the code anymore.

grfmt_tiff.hpp includes the tiff.h header inside of the tiff_dummy_namespace declaration. One implication of this is that all namespaced declarations made in tiff.h become qualified with tiff_dummy_namespace::.

Because tiff.h includes standard library headers, the std namespace declarations are converted to tiff_dummy_namespace::std declarations.

Subsequently, grfmt_tiff.hpp declares using namespace tiff_dummy_namespace;.

This can lead to an ambiguity error during the resolution of the std namespace.

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[vrabaud]

Fixes were already made in https://github.com/opencv/opencv/pull/25096

[vrabaud]

Actually, this is still problematic. Using pure C++ types is the way to go. There are only conflicts with int64 usually (defined as long long int or int64_t), which is unused in the code.