test(repro): drop external from the fast-skip invariant table

Follow-up to #2161, which removes `external` from FAST_SKIP_DIRS
(#2151). The repro board's invariant_discovery_fast_skip_dirs still
listed it among the directories MODERATE mode must skip, so with the
change the board reported "REGRESSION external not skipped in MODERATE
mode". That runner is outside the gating test target, so PR CI could
not see it.

The table matches FAST_SKIP_DIRS again. The new behaviour stays pinned
in the gating discover suite by no_skip_fast_external.

Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>

tests/repro/repro_invariant_discovery_fqn.c

@@ -288,7 +288,6 @@ TEST(invariant_discovery_fast_skip_dirs) {
         { "public" },
         { "third_party" },
         { "thirdparty" },
-        { "external" },
         { "migrations" },
         { "build" },  /* build is in FAST_SKIP_DIRS, not ALWAYS */
         { "bin" },