Remove indirection in stdio-common tests dependencies.
stdio-common/Makefile has the tests target depend on two test output
files indirectly through rules do-tst-unbputc and do-tst-printf. I
see no reason for such an indirection, and everywhere else the tests
just depend on the output files directly, so this patch removes the
indirection.
Tested x86_64.
* stdio-common/Makefile (do-tst-unbputc): Remove target.
(do-tst-printf): Likewise.
(tests): Depend directly on $(objpfx)tst-unbputc.out and
$(objpfx)tst-printf.out.