Fix: label the close-dialog draft button with the generic Save

"Save Draft" made the third button of the save-or-discard dialog wrap
onto two lines in several languages (German "Entwurf speichern", French
"Enregistrer le brouillon", ...) while its siblings stay one line. The
dialog title already says the draft is what's being saved, so the
button now uses the existing generic common.save key - one short word
in every locale, no new translations needed.

email_composer.save_draft had exactly this one consumer; the dead key
is removed from all 22 locales.

locales/en/common.json

@@ -651,7 +651,6 @@
     "continue_draft": "Continue draft",
     "close_draft_title": "Save or discard draft?",
     "close_draft_message": "You have unsaved changes. Would you like to save this as a draft or discard it?",
-    "save_draft": "Save Draft",
     "smime_sign_on": "S/MIME signing enabled",
     "smime_sign_off": "Enable S/MIME signing",
     "smime_encrypt_on": "S/MIME encryption enabled",