configure: remove check for poll.h

Introduced by:
commit b38d045dea
    Remove use of sys/poll.h on mingw

Made redundant by:
  commit 0c97e70b74
    Update event loop example programs to demonstrate best practice

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>

configure.ac

@@ -363,7 +363,6 @@ AC_CHECK_HEADERS([\
   stdarg.h \
   syslog.h \
   sys/mount.h \
-  sys/poll.h \
   sys/syscall.h \
   sys/sysctl.h \
   sys/ucred.h \