Remove PrepareAllQueries

I never used it so I think usage is probably low. Complicates the pgx v5
upgrade, so drop it.

It's easy enough to workaround by writing the prepare manually.

Additionally, it won't work well if we eventually support dynamic queries.

example/author/query.sql_test.go

@@ -80,33 +80,6 @@ func TestNewQuerier_FindAuthors(t *testing.T) {
 	})
 }
 
-func TestNewQuerier_FindAuthors_PrepareAllQueries(t *testing.T) {
-	conn, cleanup := pgtest.NewPostgresSchema(t, []string{"schema.sql"},
-		pgtest.WithGuardedStmtCache(findAuthorsSQL))
-	defer cleanup()
-	q := NewQuerier(conn)
-	adamsID := insertAuthor(t, q, "john", "adams")
-
-	t.Run("PrepareAllQueries", func(t *testing.T) {
-		err := PrepareAllQueries(context.Background(), conn)
-		require.NoError(t, err)
-	})
-
-	t.Run("FindAuthors - 1 row - john", func(t *testing.T) {
-		authors, err := q.FindAuthors(context.Background(), "john")
-		require.NoError(t, err)
-		want := []FindAuthorsRow{
-			{
-				AuthorID:  adamsID,
-				FirstName: "john",
-				LastName:  "adams",
-				Suffix:    nil,
-			},
-		}
-		assert.Equal(t, want, authors)
-	})
-}
-
 func TestNewQuerier_FindFirstNames(t *testing.T) {
 	conn, cleanup := pgtest.NewPostgresSchema(t, []string{"schema.sql"})
 	defer cleanup()