docs: remove timestamp hcl2 function (#9867)

timestamp isn't actually implemented

website/content/docs/job-specification/hcl2/functions/datetime/timestamp.mdx

@@ -1,33 +0,0 @@
----
-layout: docs
-page_title: timestamp - Functions - Configuration Language
-sidebar_title: timestamp
-description: |-
-  The timestamp function returns a string representation of the current date
-  and time.
----
-
-# `timestamp` Function
-
-`timestamp` returns the current date and time.
-
-In the Nomad language, timestamps are conventionally represented as
-strings using [RFC 3339](https://tools.ietf.org/html/rfc3339)
-"Date and Time format" syntax, and so `timestamp` returns a string
-in this format.
-
-The result of this function will change every second, so using this function
-directly with job attributes will cause a diff to be detected on every
-Nomad job submission.
-
-## Examples
-
-```shell-session
-> timestamp()
-2018-05-13T07:44:12Z
-```
-
-## Related Functions
-
-- [`formatdate`](/docs/job-specification/hcl2/functions/datetime/formatdate) can convert the resulting timestamp to
-  other date and time formats.

website/data/docs-navigation.js

@@ -241,7 +241,7 @@ export default [
               {
                 category: 'datetime',
                 name: 'Date and Time Functions',
-                content: ['formatdate', 'timeadd', 'timestamp'],
+                content: ['formatdate', 'timeadd'],
               },
               {
                 category: 'encoding',