Implement info.delete() database method.

It deletes a (possibly deeply nested) element in the
`info` table.

lib/www/server/lib/db/info/delete.js

@@ -0,0 +1,28 @@
+const { setSurvey, transaction } = require('../connection');
+
+async function del (projectId, path, opts = {}) {
+	const client = await setSurvey(projectId);
+	const [key, ...jsonpath] = (path||"").split("/").filter(i => i.length);
+
+	try {
+		const text = jsonpath.length
+			? `
+				UPDATE info
+					SET value = value #- $2
+				WHERE key = $1;
+			`
+			: `
+				DELETE FROM info
+				WHERE key = $1;
+			`;
+		const values = jsonpath.length ? [key, jsonpath] : [key];
+		await client.query(text, values);
+	} catch (err) {
+		console.error("ERROR", err);
+		throw err;
+	} finally {
+		client.release();
+	}
+}
+
+module.exports = del;