app: drop Go 1.12 support

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/datadir.go

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Unlicense OR MIT
 
-// +build !android,go1.13
+// +build !android
 
 package app
 

app/datadir_go112.go

@@ -1,12 +0,0 @@
-// SPDX-License-Identifier: Unlicense OR MIT
-
-// +build !android,!go1.13
-
-package app
-
-import "os"
-
-func dataDir() (string, error) {
-	// Use a quick workaround until we can require go 1.13.
-	return os.UserHomeDir()
-}