Remove NOT_USED BufFileTellBlock() from buffile.c

Remove NOT_USED BufFileTellBlock() from buffile.c

This routine has been marked as NOT_USED since 20ad43b576d9 from 2000,
and a patch is planned to switch the logtape/tuplestore APIs to rely on
int64 rather than long for the block nunbers, which is more portable.

Keeping it is more confusing than anything at this stage, so let's get
rid of it entirely.

Thanks for Heikki Linnakangas for the poke on this one.

Discussion: https://postgr.es/m/5047be8c-7ee6-4dd5-af76-6c916c3103b4@iki.fi