This commit deletes the direct, low-level functionality that
QElapsedTimer has carried since it was introduced. Everything now uses
only std::chrono::steady_clock and std::chrono::nanoseconds.
QDeadlineTimer temporarily still uses qt_gettime(), which is moved to
qcore_unix.cpp.
Task-number: QTBUG-110059
Change-Id: Ieec322d73c1e40ad95c8fffd174641a469b1eee5
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>