Change how links are retrieved from metadata cache

[Zachatoo]

Need to replace this undocumented Obsidian function with one from the public api.

// before
return this.app.fileManager.getAllLinkResolutions().length;

// after
let count = 0;
this.app.metadataCache.iterateReferences((sourcePath: string, reference: ReferenceCache) => {
  count++;
});
return count;

Should also remove the typescript declaration for this undocumented Obsidian function.