Remove use of op_string

Remove use of op_string

After switching to the new expression implementation, there will no
need for op_string.  Before deleting it, the one call outside of the
expression-printing code must be removed.  That is what this patch
does.

gdb/ChangeLog
2021-03-08  Tom Tromey  <tom@tromey.com>

	* ada-lang.c (ada_value_binop): Do not use op_string.