Remove transclusions in us_ticker.md

Remove transclusions from nonexistent pages to fix build.

Author: Amanda Butler

docs/reference/contributing/target/us_ticker.md

@@ -2,8 +2,6 @@
 
 Implementing the microsecond ticker enables Mbed OS to perform operations that require precise timing. You can use this API to schedule events, record elapsed time and perform submillisecond delays.
 
-<span class="warnings">**Warning:** We are changing the microsecond ticker HAL API in an upcoming release of Mbed OS. This page documents code that exists on a feature branch of Mbed OS. You can find details on how it may affect you in the [implementing the microsecond ticker API](#implementing-the-microsecond-ticker-api) section.</span>
-
 ### Assumptions
 
 #### Defined behavior
@@ -42,14 +40,10 @@ To implement this API, the device must have a hardware counter that has a count
 
 ### Implementing the microsecond ticker API
 
-We are working on the new HAL microsecond ticker API, which will replace the current version in an upcoming release of Mbed OS. You need to implement the microsecond ticker API in both variants. First, you need to implement the current API. You can find it on the master branch:
+You can find the API and specification for the microsecond ticker API in the following header file:
 
 [![View code](https://www.mbed.com/embed/?type=library)](https://os.mbed.com/docs/v5.8/mbed-os-api-doxy/us__ticker__api_8h_source.html)
 
-To make sure your platform is ready for the upcoming changes, you need to implement the future API and submit it in a separate pull request against the `feature-hal-spec-ticker` branch. You can find the API and specification for the new microsecond ticker API in the following header file:
-
-[![View code](https://www.mbed.com/embed/?type=library)](https://os.mbed.com/docs/v5.8/feature-hal-spec-ticker-doxy/group__hal__us__ticker.html)
-
 To enable microsecond ticker support in Mbed OS, add the `USTICKER` label in the `device_has` option of the target's section in the `targets.json` file.
 
 ### Testing
@@ -59,11 +53,3 @@ The Mbed OS HAL provides a set of conformance tests for the microsecond ticker.
 ```
 mbed test -t <toolchain> -m <target> -n tests-mbed_hal-common_ticker*,tests-mbed_hal-us_ticker*
 ```
-
-You can read more about the test cases:
-
- [![View code](https://www.mbed.com/embed/?type=library)](https://os.mbed.com/docs/v5.8/feature-hal-spec-ticker-doxy/group__hal__us__ticker.html)
-
- [![View code](https://www.mbed.com/embed/?type=library)](https://os.mbed.com/docs/v5.8/feature-hal-spec-ticker-doxy/group__hal__ticker__tests.html)
-
- [![View code](https://www.mbed.com/embed/?type=library)](https://os.mbed.com/docs/v5.8/feature-hal-spec-ticker-doxy/group__hal__us__ticker__tests.html)