bufmgr: Remove buffer-write-dirty tracepoints

bufmgr: Remove buffer-write-dirty tracepoints

The trace point was using the relfileno / fork / block for the to-be-read-in
buffer. Some upcoming work would make that more expensive to provide. We still
have buffer-flush-start/done, which can serve most tracing needs that
buffer-write-dirty could serve.

Discussion: https://postgr.es/m/f5164e7a-eef6-8972-75a3-8ac622ed0c6e@iki.fi