Remove env prefix from config

Environment variable prefix config allowed overriding the prefix
without changing boilerplate code.

Experience showed that these tricks are not that useful.

Also, a default prefix will probably never be the correct one,
so I decided to remove it.

cmd/modern-go-application/config.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"errors"
-	"fmt"
 	"os"
 	"strings"
 	"time"
@@ -128,10 +127,9 @@ func (c telemetryConfig) Validate() error {
 func configure(v *viper.Viper, p *pflag.FlagSet) {
 	// Viper settings
 	v.AddConfigPath(".")
-	v.AddConfigPath(fmt.Sprintf("$%s_CONFIG_DIR/", strings.ToUpper(envPrefix)))
+	v.AddConfigPath("$CONFIG_DIR/")
 
 	// Environment variable settings
-	v.SetEnvPrefix(envPrefix)
 	v.SetEnvKeyReplacer(strings.NewReplacer(".", "_", "-", "_"))
 	v.AllowEmptyEnv(true)
 	v.AutomaticEnv()

cmd/modern-go-application/vars.go

@@ -9,7 +9,4 @@ const (
 
 	// friendlyAppName is the visible name of the application.
 	friendlyAppName = "Modern Go Application"
-
-	// envPrefix is prepended to environment variables when processing configuration.
-	envPrefix = "app"
 )