Update:Remove unused missing/invalid audiobook parts logic and keys

client/strings/da.json

@@ -320,7 +320,6 @@
   "LabelIntervalEvery6Hours": "Hver 6. time",
   "LabelIntervalEveryDay": "Hver dag",
   "LabelIntervalEveryHour": "Hver time",
-  "LabelInvalidParts": "Ugyldige dele",
   "LabelInvert": "Inverter",
   "LabelItem": "Element",
   "LabelLanguage": "Sprog",
@@ -357,7 +356,6 @@
   "LabelMinute": "Minut",
   "LabelMissing": "Mangler",
   "LabelMissingEbook": "Has no ebook",
-  "LabelMissingParts": "Manglende dele",
   "LabelMissingSupplementaryEbook": "Has no supplementary ebook",
   "LabelMobileRedirectURIs": "Allowed Mobile Redirect URIs",
   "LabelMobileRedirectURIsDescription": "This is a whitelist of valid redirect URIs for mobile apps. The default one is <code>audiobookshelf://oauth</code>, which you can remove or supplement with additional URIs for third-party app integration. Using an asterisk (<code>*</code>) as the sole entry permits any URI.",