Remove the unused xRename() method from the sqlite3_vfs object.

Add better documentation on the xCurrentTimeInt64() method.

FossilOrigin-Name: 51ec0e5432dd6849b83a3d969a018482eb3bb083

src/test_devsym.c

@@ -99,7 +99,6 @@ static sqlite3_vfs devsym_vfs = {
   devsymSleep,              /* xSleep */
   devsymCurrentTime,        /* xCurrentTime */
   0,                        /* xGetLastError */
-  0,                        /* xRename */
   0                         /* xCurrentTimeInt64 */
 };