remove --nilseqs:on + associated logic to support it

[timotheecour]

rationale

• this would simplify a lot of code, eg:

  rg -i nilseqs | wc -l
    49

• nim --useVersion only supports nim >= 1.0
• nim bootstrap currently uses 0.20 and builds fine without --nilseqs:on
• the only thing that stands in the way are tests for nim in action (see testNimInAction), but IMO this isn't a viable reason to prevent cleaning up nim and reduce complexity; those tests should be updated to use what's in current nim (since even before 1.0), not what was used at the time the book was written

(tests/cpp/tasync_cpp.nim uses --nilseqs:on but doesn't need to, it works without it as jester has been patched a while ago IIRC)

[juancarlospaco]

I do not understand why a compiler needs a book in the sources. :shrug: