doc: Remove obsolete sentence about _Float* not being supported in C++ [PR106652]

As mentioned in the PR, these types are supported in C++ since GCC 13,
so we shouldn't confuse users.

2023-08-22  Jakub Jelinek  <jakub@redhat.com>

	PR c++/106652
	* doc/extend.texi (_Float<n>): Drop obsolete sentence that the
	types aren't supported in C++.