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.

(cherry picked from commit 10c6aea4434b1c9ccea30a1f87f301ab2c9bade6)