Discuss and remove std::is_floating_point specialization

[sdebionne]

Just so that this discussion doesn't get lost -event if #764 seems to fix the compile issue.

The only place where is_floating_point is used within GIL seems to be the TIFF I/O extension, and there it's only an implementation detail: https://github.com/boostorg/gil/blob/688acf78f7fc4ecb5b1c6f1e4a8f4a5939d28e9b/include/boost/gil/extension/io/tiff/detail/is_allowed.hpp#L101C13-L101C30

Furthermore I'm a bit worried, because cppreference.com states:

If the program adds specializations for std::is_floating_point or std::is_floating_point_v, the behavior is undefined.

Doesn't sound like this is something one should do. As long as those lines were in the boost namespace (before the PR), that was not a specialization of std::is_floating_point but boost::is_floating_point, a different thing - although probably with similar intentions. But with the move to the std namespace it now is a specialization.

Originally posted by @striezel in #761 (comment)