Fix recursion in debug advice

[bmag]

As reported in #182 (comment), there seemed to be some recursion caused by using cl-letf. Should be fixed by using cl-flet.

After invoking the debugger (i.e., M-x toggle-debug-on-entry RET beginning-of-buffer RET followed by M-x beginning-of-buffer RET), repeatedly stepping through (by pressing d) would recurse into our advice every time. I wasn't able to recreate the same recursion (with other test functions) outside of the debugger environment, so there is probably be another mysterious factor in the debugger that triggers the recursion.

[wyuenho]

This unfortunately doesn't work for my use case as we are back to pop-to-buffer not being able to pick a window wide enough again. Judging from the different heights of the windows in every invocation of debug, I don't think this advice has any effect whatsoever. With that said, I think it's best to revert #182 for now. I've since discovered that although it can hand control to purpose to pick a window, it doesn't pick a frame, so if toggle-debug-on-error is turned on and an error is thrown inside a small child frame like those made by company-box, the backtrace buffer is still popped up inside that tiny child frame. Picking a right window from the right frame needs a re-think.

[bmag]

Ok, reverted (in 06f727e)