Simplify and tune XSYMBOL

Simplify and tune XSYMBOL

* src/lisp.h (XSYMBOL): Simplify and tune.  There is no need to
examine symbols_with_pos_enabled here, since the arg must be a symbol
so if it's not a bare symbol then it must be a symbol_with_pos;
and checking whether a symbol is bare is cheap.

With Ubuntu 23.10 on a Xeon W-1350, this shrank Emacs’s executable
text size by 0.1% and sped up a default build of all *.elc files by
0.4%.

Remove unnecessary eassert, since XBARE_SYMBOL and XSYMBOL_WITH_POS
have easserts that suffice.