PATCH: AR-3091 Login form "Email or username" label incorrect

Our hack to the login form currently only supports identifying the user
by email address, not by username.  Until that is fixed, we should change
the field label to read "Email address" rather than "Email or username".

Concurrent with this patch, we should remove the custom configuration
for `Accounts_EmailOrUsernamePlaceholder`, returning it to the default empty string.
We had customized it "Email address" when 5.x treated the empty string as "Email or username".
However 6.x treats the empty string as "example@example.com".

apps/meteor/packages/rocketchat-i18n/i18n/en.i18n.json

@@ -5948,7 +5948,7 @@
   "registration.component.login.incorrectPassword": "Incorrect password",
   "registration.component.switchLanguage": "Change to <1>{{name}}</1>",
   "registration.component.resetPassword": "Reset password",
-  "registration.component.form.emailOrUsername": "Email or username",
+  "registration.component.form.emailOrUsername": "Email address",
   "registration.component.form.username": "Username",
   "registration.component.form.name": "Name",
   "registration.component.form.nameOptional": "Name optional",