Remove SYMBOL_WITH_POS_{POS,SYM}

Remove SYMBOL_WITH_POS_{POS,SYM}

* src/fns.c (internal_equal): Turn comment into eassert
that !symbols_with_pos_enabled.
(sxhash_obj): Simplify case of symbol with pos (when enabled).
* src/lisp.h (XSYMBOL_WITH_POS_SYM, XSYMBOL_WITH_POS_POS)
(maybe_remove_pos_from_symbol): New inline functions.
(SYMBOL_WITH_POS_SYM, SYMBOL_WITH_POS_POS): Remove.
All uses replaced by the new functions.  This avoids some
double-checking in the source code, simplifies the code overall,
and avoids the need for "Type checking is done in the following
macro" comments to explain unusual code.