t/check-non-portable-shell: stop being so polite

Error messages emitted by this linting script are long and noisy,
consisting of several sections:

<test-script>:<line#>: error: <explanation>: <failed-shell-text>

Many problem explanations ask the reader to "please" use a suggested
alternative, however, such politeness is unnecessary and just adds to
the noise and length of the line, so drop "please" to make the message a
bit more concise.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>