Remove two quick_symbol_functions methods

Remove two quick_symbol_functions methods

quick_symbol_functions::read_partial_symbols is no longer implemented,
so both it and quick_symbol_functions::can_lazily_read_symbols can be
removed.  This allows for other functions to be removed as well.

Note that SYMFILE_NO_READ is now pretty much dead.  I haven't removed
it here -- but could if that's desirable.  I tend to think that this
functionality would be better implemented in the core; but whenever I
dive into the non-DWARF readers it is pretty depressing.