Revert "Load environment variables as a resolver (#480)"

This reverts commit 3cedc44821.

Fixes #497, #498

model.go

@@ -3,6 +3,7 @@ package kong
 import (
 	"fmt"
 	"math"
+	"os"
 	"reflect"
 	"strconv"
 	"strings"
@@ -376,6 +377,19 @@ func (v *Value) ApplyDefault() error {
 // Does not include resolvers.
 func (v *Value) Reset() error {
 	v.Target.Set(reflect.Zero(v.Target.Type()))
+	if len(v.Tag.Envs) != 0 {
+		for _, env := range v.Tag.Envs {
+			envar, ok := os.LookupEnv(env)
+			// Parse the first non-empty ENV in the list
+			if ok {
+				err := v.Parse(ScanFromTokens(Token{Type: FlagValueToken, Value: envar}), v.Target)
+				if err != nil {
+					return fmt.Errorf("%s (from envar %s=%q)", err, env, envar)
+				}
+				return nil
+			}
+		}
+	}
 	if v.HasDefault {
 		return v.Parse(ScanFromTokens(Token{Type: FlagValueToken, Value: v.Default}), v.Target)
 	}