doc: develop: toolchains: Remove 'xtools' documentation

The `xtools` toolchain variant has been deprecated since Zephyr v3.3.0 and
now removed.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

doc/develop/test/twister.rst

@@ -116,7 +116,6 @@ required for best test coverage for this specific board:
   toolchain:
     - zephyr
     - gnuarmemb
-    - xtools
   supported:
     - arduino_gpio
     - arduino_i2c

doc/develop/toolchains/crosstool_ng.rst

@@ -16,7 +16,6 @@ Guides on how to set up toolchains for Zephyr development.
    gnu_arm_embedded.rst
    intel_oneapi_toolkit.rst
 
-   crosstool_ng.rst
    host.rst
    other_x_compilers.rst
    custom_cmake.rst

doc/develop/toolchains/index.rst

@@ -186,7 +186,7 @@ Removed APIs in this release
 Deprecated in this release
 ==========================
 
-* :ref:`xtools toolchain variant <toolchain_xtools>` is now deprecated. When using a
+* ``xtools`` toolchain variant is now deprecated. When using a
   custom toolchain built with Crosstool-NG, the
   :ref:`cross-compile toolchain variant <other_x_compilers>` should be used instead.