Remove obsolete note from Title->canExist()

A bit of history:
* when added by Aaron Schulz with commit
  df1f744757b0ce695e6aa1e4a28f6c50fe6d8e20, the method made use of
  a hardcoded `0`;
* Lisa Ridley noted that with
  commit bc1023ac89f9214c6510d97869fb4f280084995f by adding
  "uses hardcoded namespace index instead of constants";
* Max Semenik changed `0` to `NS_MAIN`, thus making the comment
  obsolete, with commit a35fe1f5f2f8eda52a93eb974516c3b65918c819.

Change-Id: I0c2dad773219da80cc4db04ad6a8007a03207e5c
Merge "Remove obsolete note from Title->canExist()"