apps: drop redundant 64-bit casts on tv_sec/tv_nsec arithmetic

[xiaoxiang781216]

Summary

Follow-up cleanup to the recent time_t/clock_t 64-bit consolidation
(commits "drop CONFIG_SYSTEM_TIME64 conditionals" and
"replace sclock_t with clock_t and drop redundant time_t/off_t casts").

NuttX's time_t is now unconditionally a signed 64-bit type, so the
explicit (uint64_t), (int64_t), and (int) casts that used to be
sprinkled around tv_sec / tv_nsec arithmetic — to avoid 32-bit
multiplication/subtraction overflow back when time_t could be 32-bit —
are no longer needed. This PR drops those now-redundant casts to keep
the timekeeping helpers readable and consistent with the rest of the
recently-cleaned-up code.

Files touched (one-liners each):

• benchmarks/cyclictest/cyclictest.c — timediff_us()
• benchmarks/sd_bench/sd_bench_main.c — get_time_delta_us()
• examples/oneshot/oneshot_main.c — maxus computation
• examples/watchdog/watchdog_main.c — current_time_ms (×2)
• industry/nxmodbus/nxmb_internal.h — nxmb_util_clock_ms()
• netutils/ntpclient/ntpclient.c — timespec2ntp()
• netutils/ptpd/ptpd.c — ptp_adjtime()
• system/dd/dd_main.c — elapsed accounting
• testing/drivers/drivertest/drivertest_posix_timer.c — get_timestamp()
• testing/drivers/sd_stress/sd_stress_main.c — get_time_delta()
• testing/sched/getprime/getprime_main.c — elapsed accounting
• testing/sched/pthread_mutex_perf/pthread_mutex_perf.c — timespec_avg()

Impact

• Users: none — purely a readability/cleanup change.
• Build process: none — same generated code on every supported
  compiler (the dropped casts were already no-ops once time_t
  became 64-bit).
• Hardware: none.
• Documentation: none.
• Security: none.
• Compatibility: none — time_t/tv_sec/tv_nsec widths are
  unchanged; this PR only removes pleonastic casts.

No behavioural change is intended.

Testing

Build-tested host-side together with the corresponding nuttx-side
time_t/clock_t cleanups; the touched helpers all expand to the
same arithmetic as before because the dropped casts no longer
widen anything.

• Host: Ubuntu 22.04 x86_64, GCC 11
• Targets exercised by the changed sources at build time:
  • sim:nsh (covers examples/watchdog, examples/oneshot,
    system/dd, testing/sched/*, benchmarks/cyclictest,
    benchmarks/sd_bench, testing/drivers/*)
  • sim:netnsh (covers netutils/ntpclient, netutils/ptpd,
    industry/nxmodbus)

No new compiler warnings; no functional difference observed in the
benchmark / test outputs before and after the change.

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[cederom]

Thank you @xiaoxiang781216 :-) CI failed, restarted :-)

There is small typo to fix in nuttx-apps/apps/testing/drivers/sd_stress/sd_stress_main.c:137: bufffer ==> buffer