Remove unused functions in regress.c.

Remove unused functions in regress.c.

This patch removes five functions that presumably were once used in the
regression tests, but haven't been so used in many years.  Nonetheless
we've been wasting maintenance effort on them (e.g., by converting them
to V1 function protocol).  I see no reason to think that reviving them
would add any useful test coverage, so drop 'em.

In passing, mark regress_lseg_construct static, since it's not called
from outside this file.

Discussion: https://postgr.es/m/29322.1519701006@sss.pgh.pa.us