Remove cleanups from gdb_readline_wrapper

Remove cleanups from gdb_readline_wrapper

This removes some cleanups from gdb_readline_wrapper by changing the
existing gdb_readline_wrapper_cleanup struct to have a constructor and
destructor, and then changing gdb_readline_wrapper to simply
instantiate it on the stack.

gdb/ChangeLog
2018-03-27  Pedro Alves  <palves@redhat.com>
	    Tom Tromey  <tom@tromey.com>

	* top.c (class gdb_readline_wrapper_cleanup): Add constructor,
	destructor.  Now a class.
	(gdb_readline_wrapper_cleanup): Remove function.
	(gdb_readline_wrapper): Remove cleanups.