tbuffer_t no longer exists on VMS.

It was replaced by the standard tms struct in v7.0, released in
1995.  Explicit support for pre-7.0 was removed in 32995a382d65b
for Perl 5.16, but I missed the tbuffer_t bit, which tripped up
Nicholas in 25983af42cdcf2dc, because he asked for:

  struct tbuffer_t

which via macro expansion became:

  struct struct tms

which failed to compile.  So remove code that's unnecessarily
different on VMS, leaving only a tbuffer_t compatibility macro
with a more appropriate comment so it will hopefully be less
likely to get used in new code.