Replace uint64_type by std::uint64_t

[SunBlack]

Removed additionally checks for size of uint64_type, as result will be always true, as this type is defined as:

unsigned integer type with width of exactly 8, 16, 32 and 64 bits respectively
(provided only if the implementation directly supports the type)

This fixes current compile issues due to bad PR with sed s/uint64_t/std::uint64_t for uint64_type before.

[SergioRAgostinho]

@PointCloudLibrary/maintainers allow API breakage here since this is an unstable module?

[kunaltyagi]

Maybe deprecate it, and not use it. Like with pcl::int_t types

[SunBlack]

Name of file is internal.h - this doesn't sound as it is intended to be used outside.

[SergioRAgostinho]

Maybe deprecate it, and not use it. Like with pcl::int_t types

It is relevant to mention, that I have not been able to compile PCLs CUDA code with C++14 flags. So the usual deprecated attribute might be off-limits and we got rid of our custom deprecation pragmas.

[kunaltyagi]

Putting a line in CHANGELOG (and/or RELEASE_NOTES) should be sufficient since this is an internal API and that is allowed to change without warnings.