Remove obsolete xtools toolchain variant

[stephanosio]

Is your enhancement proposal related to a problem? Please describe.

The xtools toolchain variant (aka. Crosstool-NG) was originally intended to be used for the then-"new generation" of Crosstool-NG based Zephyr SDK toolchains (aka. sdk-ng) -- see https://docs.zephyrproject.org/latest/develop/toolchains/crosstool_ng.html.

This is no longer necessary because the current Zephyr SDK (sdk-ng) already has its own toolchain type (i.e. zephyr), which fully replaces the xtools toolchain variant, and the xtools toolchain variant serves no purpose at all.

Moreover, keeping the xtools toolchain variant creates more problems because the Zephyr SDK build system integration is now provided on the Zephyr SDK side as a CMake package, while this xtools integration lives in the main Zephyr repository -- resulting in a synchronisation problem.

Describe the solution you'd like

Remove the xtools toolchain variant since it is obsolete and serves no purpose.

Describe alternatives you've considered

No other sensible alternative exists.

[tejlmand]

Fixed by: #80466