Eliminate AMGf_set flag

Eliminate AMGf_set flag

I added this flag a few years ago when I revamped the overload macros
tryAMAGICbin() etc. It allowed two different classes of macros to
share the same functions (Perl_try_amagic_un/Perl_try_amagic_bin)
by indicating what type of action is required.

However, the last few commits have made those two functions able to
robustly always determine whether its an assign-type action
($x op= $y or  $lex = $x op $x) or a plain set-result-on-stack operation
($x op $y).

So eliminate this flag.

Note that this makes the ops which have the AMGf_set flag hard-coded
infinitesimally slower, since Perl_try_amagic_bin no longer skips the
checks for assign-ness. But compared with the overhead of having
already called the overload method, this is is trivial.

On the plus side, it makes the code smaller and easier to understand.