gdb: remove some dead code from completer.c

In completer.c there is some code that is surrounded with '#if 0',
this code:

  #if 0
    /* There is no way to do this just long enough to affect quote
       inserting without also affecting the next completion.  This
       should be fixed in readline.  FIXME.  */
    /* Ensure that readline does the right thing
       with respect to inserting quotes.  */
    rl_completer_word_break_characters = "";
  #endif

This code, in some form, and always defined out, has been around since
the original import of GDB.  Though the comment hints at what the
problem might be, it's not really clear what the issue is.  And
completion within GDB has moved on a long way since this code was
written ... but not used.

I'm proposing that we just remove this code.

If/when a problem comes up then we can look at how to solve it.  Maybe
this code would be the answer ... but also, I suspect, given all the
changes ... maybe not.  I'm not sure carrying around this code for
another 20+ years adds much value.

There should be no user visible changes after this commit.