Remove ENSURE-ONE-OF from fd-stream.

It's now clear how this would "Allow the user to fix any problems"
since the call was to ERROR and not CERROR. But that aside,
you'd never reach it - the arg types are precisely declared in fndb.
More reader cruft deletion.

- The UNDEFINED-MACRO-CHAR function is never installed
  as a macro function. All constituents were made to have
  a macro entry of NIL in changes
   0e43dbcc9599e1806d062703b69ebeb131a0f706 and
   cc676f35baa0a46df06d9917e087ca466d053027

- READTABLE constructor takes no arguments.

- Random comment fixes.