t9903-bash-prompt: don't check the stderr of __git_ps1()

A test in 't9903-bash-prompt.sh' fails when the test script is run with '-x' tracing and a Bash version not yet supporting BASH_XTRACEFD, notably the default Bash version shipped in OSX. The reason for the failure is that the test checks the emptiness of __git_ps1()'s stderr, which includes the trace of all commands executed within __git_ps1() as well, throwing off the emptiness check. Having only a single test checking the empty stderr doesn't bring us much when none of the other tests do so, so remove this test for now. After this change t9903 passes with '-x', even when running with a Bash version not yet supporing BASH_XTRACEFD. In the future we might want to consider checking the emptiness of __git_ps1()'s stderr in each and every test, in which case we'd have to mark this test script as 'test_untraceable', but that's a different topic. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>