clean: do not use strbuf_split*() [part 2]

clean: do not use strbuf_split*() [part 2]

builtin/clean.c:filter_by_patterns_cmd() interactively reads a line
that has exclude patterns from the user and splits the line into a
list of patterns.  It uses the strbuf_split() so that each split
piece can then trimmed.

There is no need to use strbuf anymore, thanks to the recent
enhancement to string_list_split*() family that allows us to trim
the pieces split into a string_list.

Signed-off-by: Junio C Hamano <gitster@pobox.com>