PIPE-3088: remove redundant me.doframe(obs_pos) from inner loop
The observatory position frame is already set once before all loops.
Calls to me.doframe() for the epoch type do not affect the
observatory frame, so re-doing doframe(obs_pos) on every timestamp
is unnecessary.
Timing (compute_zd_telmjd_for_ms, 50 runs, 74 timestamps):
before (d851488da): 12.629 ms per call
after (this commit): 9.204 ms per call
speedup: 1.37x (27% reduction)
Cumulative vs main (22.901 ms): 2.49x speedup
Exact output preserved (verified by test_telmjd_exact).
