JS: remove hack in RTS to support old cabal

Older cabal wouldn't build a library composed only of js-sources. So we used the following hack in rts.cabal:

      -- dummy file to force the build of a .a lib
      -- FIXME (Luite, 2022-08) do we still need the c-sources file?
      c-sources: version.c

The task is to remove this empty C file now that we use a cabal that has been fixed.