fix: do not set shadowDatabaseUrl in schema.prisma

I ran into issues with the `prisma migrate` command:

```
Error: P3005

The database schema is not empty.
```

After some research, I was able to resolve the issue by removing the
`shadowDatabaseUrl` from `schema.prisma`.

> Important: Do not use the same values for url and shadowDatabaseUrl.
https://www.prisma.io/docs/concepts/components/prisma-migrate/shadow-database

> Do _not_ use `shadowDatabaseUrl` with Vercel Postgres any more,
> especially not with the same value as `url` or `directUrl`.
> The shadow database can be created dynamically.
prisma/prisma#19234 (comment)

See prisma/prisma#19234

prisma/schema.prisma

@@ -4,12 +4,11 @@ generator client {
 }
 
 datasource db {
-  provider          = "postgresql"
-  url               = env("POSTGRES_PRISMA_URL") // uses connection pooling
-  directUrl         = env("POSTGRES_URL_NON_POOLING") // uses a direct connection
-  shadowDatabaseUrl = env("POSTGRES_URL_NON_POOLING") // used for migrations
-  schemas           = ["events", "portal", "profiles", "storage"]
-  extensions        = [pg_trgm, postgis]
+  provider   = "postgresql"
+  url        = env("POSTGRES_PRISMA_URL") // uses connection pooling
+  directUrl  = env("POSTGRES_URL_NON_POOLING") // uses a direct connection
+  schemas    = ["events", "portal", "profiles", "storage"]
+  extensions = [pg_trgm, postgis]
 }
 
 enum ProfileType {