Remove custom redundant full page write description from GIN

Hi hackers!
While reading waldump output for the GIN index I noticed $subj.

Here is an example:

```
rmgr: Gin         len (rec/tot):     53/   113, tx:        788, lsn:
0/01D2B700, prev 0/01D2B590, desc: INSERT isdata: F isleaf: T (full
page image), blkref #0: rel 1663/16384/32857 blk 1 FPW
```

Notice we have "(full page image)" from gindesc and "FPW" from generic
waldump code

I suggest removing this custom FPW support.

-- 
Best regards,
Kirill Reshke