RTEMS | bsps/powerpc/beatnik: Remove obsolete installation instructions from README (#5408)

[Ayush Daga (@dagaayush1205)]

Ayush Daga created an issue: https://gitlab.rtems.org/rtems/rtos/rtems/-/issues/5408



## Summary

The README.md in `bsps/powerpc/beatnik` contains outdated instructions referring to a `beatnik.tgz` archive and manual patching of `acinclude.ac`.

Since the BSP is now integrated into the mainline tree, these instructions are misleading and should be removed or updated to reflect the current build system (waf).

Reference: Pointed out by Joel Sherrill during review of !862.

## Steps to reproduce

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