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

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>