build: drop unused feature-detection code for Apple `poll()`

build: drop unused feature-detection code for Apple `poll()`

Drop Apple-specific detection logic for `poll()`. This detection snippet
has been disabled for Apple in both configure and cmake, for `poll()`
being broken on Apple since 10.12 Sierra (2016).

Also replace `exit(1);` with `return 1;` in configure, to make the
snippets match.

Added in 9297ca49f5f3caca938a679b9c1feeb719e61ddb #1057 (2016-10-11).

Disabled for:
configure/darwin in a34c7ce7546f39f24692925d66c2f42324dd94e9 (2016-10-18)
cmake/macOS in 825911be587bbabc3b7e4777ed3bd1bb7c858b58 #7619
cmake/iOS in d14831233df3a15b14db563156614c9ea60fcf06 #8244
cmake/all Apple in a86254b39307af1a53735b065a382567805cd9b8 #12515

Closes #14718