Remove an obsolete TODO comment from several minidump implementations.

The comment referenced a non-existent function, and these minidump
implementations already buffer discontiguous physical data pages by
mapping them into a single VA range that gets passed to the dump device,
so there is no real advantage in batching calls to blk_write().

The RISC-V and MIPS minidump implementations still write a page at a
time and so would benefit from some form of batching.

MFC after: 2 weeks
Sponsored by: Juniper Networks, Klara Inc.