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/ko/common.json

@@ -648,7 +648,6 @@
     "continue_draft": "이어서 작성하기",
     "close_draft_title": "임시보관함에 저장할까요?",
     "close_draft_message": "저장되지 않은 내용이 있어요. 임시보관함에 저장할까요, 아니면 삭제할까요?",
-    "save_draft": "임시보관",
     "smime_sign_on": "S/MIME 서명 켜짐",
     "smime_sign_off": "S/MIME 서명 켜기",
     "smime_encrypt_on": "S/MIME 암호화 켜짐",