Remove duplicate initialization in initialize_brin_buildstate().

Remove duplicate initialization in initialize_brin_buildstate().

Commit dae761a added initialization of some BrinBuildState fields
in initialize_brin_buildstate(). Later, commit b437571 inadvertently
added the same initialization again.

This commit removes that redundant initialization. No behavioral
change is intended.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Shinya Kato <shinya11.kato@gmail.com>
Discussion: https://postgr.es/m/CAEoWx2nmrca6-9SNChDvRYD6+r==fs9qg5J93kahS7vpoq8QVg@mail.gmail.com