Some code cleanup.  There are five uses of the WT_WITH_BUCKET_STORAGE macro at present.  Now that we use the WT_FILE_OPENER abstraction to open files, we only need to use the WT_WITH_BUCKET_STORAGE macro in tiered_opener.c, and not elsewhere.  It's possible we'll need it for dealing with non-Btrees that may be stored in shared storage, like WiredTiger.turtle, but we can deal with that when we implement it.