BUILD: poll: remove unused hap_fd_isset() which causes a warning with clang

BUILD: poll: remove unused hap_fd_isset() which causes a warning with clang

Clang reports that this function is not used :

src/ev_poll.c:34:28: warning: unused function 'hap_fd_isset' [-Wunused-function]
static inline unsigned int hap_fd_isset(int fd, unsigned int *evts)

It's been true since the rework of the pollers in 1.5 and it's unlikely we'll
ever need it anymore, so better remove it now to provide clean builds.

This fix can be backported to 1.6 and 1.5 now.