libc: Remove futimens() and utimensat() compat stubs.

The futimens() and utimensat() compat stubs allowed using these functions on
kernels that did not have the system calls yet (10.2, old 11-current).

Also remove the documentation of the [ENOTSUP] error that could occur with
an old kernel.

A -DNO_CLEAN build may fail because the depend files refer to the deleted
files.