t5510: consolidate 'grep' and 'test_i18ngrep' patterns

t5510: consolidate 'grep' and 'test_i18ngrep' patterns

One of the tests in 't5510-fetch.sh' checks the output of 'git fetch'
using 'test_i18ngrep', and while doing so it prefilters the output
with 'grep' before piping the result into 'test_i18ngrep'.

This prefiltering is unnecessary, with the appropriate pattern
'test_i18ngrep' can do it all by itself.  Furthermore, piping data
into 'test_i18ngrep' will interfere with the linting that will be
added in a later patch.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>