internal/postgres: remove rows from documentation before inserting
internal/postgres: remove rows from documentation before inserting

The documentation table has obsolete rows now that we are using
all/all to represent documentation for all build contexts.
So remove existing rows for a unit before inserting new ones.

For golang/go#37232

Change-Id: Icee1457ecc1e336c42e66e16d58a1f802a6c95f6
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/289989
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite