configure: simplify exit traps

This does the same thing and also fixes the trapping in
some (possibly broken) shells.

Suggested by Michael Kostylev.
Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -1810,7 +1810,6 @@ tmpfile(){
 }
 
 trap 'rm -f -- $TMPFILES' EXIT
-trap exit HUP INT TERM
 
 tmpfile TMPC  .c
 tmpfile TMPE  $EXESUF