strbuf: remove prefixcmp() and suffixcmp()

As starts_with() and ends_with() have been used to
replace prefixcmp() and suffixcmp() respectively,
we can now remove them.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'cc/starts-n-ends-with-endgame' into next

Endgame for the cc/starts-n-ends-with topic; this needs to be
evil-merged with other topics that introduce new uses of
prefix/suffix-cmp functions.

* cc/starts-n-ends-with-endgame:
  strbuf: remove prefixcmp() and suffixcmp()