headers: Remove the unused ifdefs for USE_NO_MINGW_SETJMP_TWO_ARGS

This was added in the form of USE_MINGW_SETJMP_TWO_ARGS in
6ef8b92929cc918dc50fb6802839be61c0cfab7c in 2008, then changed
so that USE_MINGW_SETJMP_TWO_ARGS was defined automatically for
64 bit targets in 5b99e86d446ae5971be734c1fa76a21c3adb7148 one
day later.

In 82347ded0d43a80de68b6a35a209717bded5f28a in 2009, i386 targets
were changed to use _setjmp3 instead of _setjmp. In this change,
the automatic setting of USE_MINGW_SETJMP_TWO_ARGS was removed and
the ifdef was inverted into USE_NO_MINGW_SETJMP_TWO_ARGS. This
left the old codepaths unused unless the user set
USE_NO_MINGW_SETJMP_TWO_ARGS manually.

To the best of our knowledge, no users have been using
USE_NO_MINGW_SETJMP_TWO_ARGS, and it only would have worked on i386
targets anyway. Thus remove this to reduce unnecessary complexity
in this otherwise complex header.

This change looks like a large change unless the diff viewer is
instructed to ignore whitespace changes (with e.g. "git show -w");
when removing the outer ifdef I reindent the inner preprocessor
defines and clarify the indentation structure in places with
mixed indentation step sizes.

Signed-off-by: Martin Storsjö <martin@martin.st>