Remove removed files from ext/.document [ci skip]

- dbm/dbm.c removed at 55cd3e4ebff8fa75854ecadcd77abbf7cf4b5823
- fiber/fiber.c removed at 521ad9a13aef1e1dae4eaff70d3e6b2e358aa095
- gdbm/gdbm.c removed at edcc29dcff1b269b7748ab83adf21b2f3f97ebff

ext/.document

@@ -11,7 +11,6 @@ date/date_parse.c
 date/date_strftime.c
 date/date_strptime.c
 date/lib
-dbm/dbm.c
 digest/bubblebabble/bubblebabble.c
 digest/digest.c
 digest/lib
@@ -22,7 +21,6 @@ digest/sha2/sha2init.c
 digest/sha2/lib
 etc/etc.c
 fcntl/fcntl.c
-fiber/fiber.c
 fiddle/closure.c
 fiddle/conversions.c
 fiddle/fiddle.c
@@ -31,7 +29,6 @@ fiddle/pinned.c
 fiddle/pointer.c
 fiddle/handle.c
 fiddle/lib
-gdbm/gdbm.c
 io/console/console.c
 io/console/lib
 io/nonblock/nonblock.c