plugin/health: cleanups (#2811)

Small, trivial cleanup: got triggered because I saw a comment on how
health plugins polls other plugins which isn't true.

* Remove useless newHealth function
* healthParse -> parse
* Remove useless constants

Net deletion of code.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/health/setup_test.go

@@ -30,7 +30,7 @@ func TestSetupHealth(t *testing.T) {
 
 	for i, test := range tests {
 		c := caddy.NewTestController("dns", test.input)
-		_, _, err := healthParse(c)
+		_, _, err := parse(c)
 
 		if test.shouldErr && err == nil {
 			t.Errorf("Test %d: Expected error but found none for input %s", i, test.input)