Revert "migrations: add num_imports column to units table"

This reverts commit c7fc077.

Reason for revert: migration times out

Change-Id: I2d41309f19463226cfca0c97940d6ab464f73ce0
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/721100
kokoro-CI: kokoro <noreply+kokoro@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

Author: ethanalee-work