cp: Remove useless ssize_t cast

Both wcount and wresid are ssize_t so this cast is not needed. Just
remove it so the code is easier to read.

Signed-off-by: Collin Funk <collin.funk1@gmail.com>
Reviewed by: emaste, zlei
Pull Request: https://github.com/freebsd/freebsd-src/pull/1116

(cherry picked from commit 8b864151856fbba97fed16b1200248cf9f6f3ccf)