test: delete dead @test gates 910/997 — coverage re-hosted in E2-C2 (M4 E3-C2, #22)

910_at_test.c and 997_at_test_ww.c build via `ww build` then read
<stem>.combined.ww as a driver side-effect; the E3 flip removes that
side-effect, so they cannot pass post-flip. Their coverage was fully
re-hosted in E2-C2 -- every axis (run-exit, -T cs/ww byte-id, the
userman/badsig/undefbody/linkfail rejects, the dup-decl family, modfn
coexist, user `fn run` coexist) has a live home in 989_septest_run,
911_attest_drop, or 989_declns_sep, several strengthened. Deleting them
before the flip keeps `make test` bisect-clean.

Also retargets a stale 'Sibling of 910_at_test' doc comment in
911_attest_record.c to the surviving gates.

test/wcc/911_attest_record.c

@@ -1,8 +1,8 @@
 /*
  * 911_attest_record — #17 record-and-continue harness (lib/test + the
- * table-synth -T entry). Sibling of 910_at_test (which pins the synth
- * itself + the #6 non-T drop); this one pins the RUNTIME behaviour the
- * fork+wait4 runner adds:
+ * table-synth -T entry). Sibling of 989_septest_run + 911_attest_drop
+ * (which pin the synth entry itself + the #6 non-T drop); this one pins
+ * the RUNTIME behaviour the fork+wait4 runner adds:
  *
  *   1. RECORD-AND-CONTINUE — `ww test` the mixed fixture (one pass, then
  *      assert-fail/SIGSEGV/SIGFPE, then a final pass). The runner must