bpo-45723: Remove obsolete AC_EXEEXT from configure.ac (GH-29486)

From the autoconf docs *Obsolete Macros* section:

    Defined the output variable EXEEXT based on the output of the 
    compiler, which is now done automatically. Typically set to empty
    string if Posix and ‘.exe’ if a DOS variant.

Author: Erlend Egeberg Aasland

configure

@@ -5738,7 +5738,6 @@ atheos*|Linux*/1*)
    exit 1;;
 esac
 
-
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for --with-suffix" >&5
 $as_echo_n "checking for --with-suffix... " >&6; }
 

configure.ac

@@ -947,7 +947,6 @@ atheos*|Linux*/1*)
    exit 1;;
 esac
 
-AC_EXEEXT
 AC_MSG_CHECKING(for --with-suffix)
 AC_ARG_WITH(suffix,
             AS_HELP_STRING([--with-suffix=SUFFIX], [set executable suffix to SUFFIX (default is '.exe')]),