sql: do not show FAMILY if there is only one on SHOW CREATE TABLE

[otan]

Release note (sql change): SHOW CREATE TABLE no longer shows the FAMILY
clause if there is only the PRIMARY family clause.

[cockroach-teamcity]

This change is 

[rafiss]

lgtm if we get PM signoff

Reviewed all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @otan and @rafiss)

---

pkg/sql/show_create_clauses.go, line 402 at r1 (raw file):

	// it is named "primary".
	families := desc.GetFamilies()
	if len(families) == 1 && families[0].Name == "primary" {

i think there's a const we can use instead of hard-coding the string

[adityamaru]

Bulk changes LGTM!

[otan]

ok has been given

bors r=rafiss

[craig]

Build succeeded:

• GitHub CI (Cockroach)