`string-{prefix,match}-p' (compat): remove

As stated in commit dc72fc03, the `string-prefix-p' and `string-match-p'
compatibility functions no longer serve any purpose since we decided to
drop support for Emacs < v23.2 (see commit 83dac351 and issue #708).

Signed-off-by: Pieter Praet <pieter@praet.org>