Translations: fix vanishing translation

Since 1.1.0 translations are saved per section. Thirtybees first clears
all translation from given section, and then import new translations
submitted in form.

The clearing mechanism works on prefix bases. If other section starts
with the same prefix, its entries would be removed as well. For example,
saving `authentication` section would remove all translations from
sections `authentication-login` or `authentication-create`.

The fix is simple -- don't clear translations at all. Remove only
entries if the input post value contain empty string.

Closes thirtybees#976

controllers/admin/AdminTranslationsController.php

@@ -1326,22 +1326,12 @@ protected function writeTranslationFile($overrideFile = false)
             $_POST['type']
         );
 
-        // To deal with vanished translations, remove all translations
-        // belonging to the saved panel before adding the ones POSTed.
-        $keyBase = array_keys($_POST)[0];
-        if ($keyBase) {
-            $keyBase = substr($keyBase, 0, strrpos($keyBase, '_'));
-            foreach (array_keys($translationsArray) as $key) {
-                if (strpos($key, $keyBase) === 0 /* start of string! */) {
-                    unset($translationsArray[$key]);
-                }
-            }
-        }
-
-        // Get all POST which aren't empty
+        // update translations
         foreach ($_POST as $key => $value) {
             if (!empty($value)) {
                 $translationsArray[$key] = $value;
+            } else {
+                unset($translationsArray[$key]);
             }
         }