Revert "Add AC_SUBST(PYTHON) so that the PYTHON var can be used by the build"

This reverts commit ff2fec6aac243bb3ea7380709212454c3bedcc18.

configure.in

@@ -4178,8 +4178,7 @@ fi
 AC_SUBST(SYSTEM_PYTHON)
 AC_SUBST(PYTHON_CFLAGS)
 AC_SUBST(PYTHON_LIBS)
-AC_SUBST(PYTHON)
-HOME=`echo $HOME | $SED 's:\\\\:/:g'`
+HOME=`echo $HOME | sed 's:\\\\:/:g'`
 AC_SUBST(HOME)
 
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