__fish_apropos: assume only /usr/bin/apropos is broken

Also, remove a macOS version check that's doom to obsolescence,
and extract a function (WIP).

share/functions/__fish_without_manpager.fish

@@ -0,0 +1,7 @@
+function __fish_without_manpager
+    MANPAGER=cat WHATISPAGER=cat if test "$argv[1]" = command
+        command $argv[2..]
+    else
+        $argv
+    end
+end