remove unnecessary table_open/close from makeArrayTypeName

Commit bc8036fc666a (12 years ago) seems to have introduced an
unnecessary catalog heap_open/close in order to do syscache accesses.
I presume a preliminary version of the patch used sysscans or something.
I don't think that's necessary, so this patch removes it.

(I noticed while reading Paul Jungwirth's patch that changes how this
works.)

-- 
Álvaro Herrera                         Developer, https://www.PostgreSQL.org/