Remove deprecated buildbot scripts

Tools/buildbot/build-amd64.bat

-@rem Formerly used by the buildbot "compile" step.
-@echo This script is no longer used and may be removed in the future.
-@echo To get the same effect as this script, use
-@echo     PCbuild\build.bat -d -e -k -p x64
-call "%~dp0build.bat" -p x64 %*

Tools/buildbot/clean-amd64.bat

-@rem Formerly used by the buildbot "clean" step.
-@echo This script is no longer used and may be removed in the future.
-@echo To get the same effect as this script, use `clean.bat` from this
-@echo directory and pass `-p x64` as two arguments.
-call "%~dp0clean.bat" -p x64 %*

Tools/buildbot/external-amd64.bat

-@echo This script is no longer used and may be removed in the future.
-@echo Please use PCbuild\get_externals.bat instead.
-@"%~dp0..\..\PCbuild\get_externals.bat" %*

Tools/buildbot/external.bat

-@echo This script is no longer used and may be removed in the future.
-@echo Please use PCbuild\get_externals.bat instead.
-@"%~dp0..\..\PCbuild\get_externals.bat" %*

Tools/buildbot/test-amd64.bat

-@rem Formerly used by the buildbot "test" step.
-@echo This script is no longer used and may be removed in the future.
-@echo To get the same effect as this script, use
-@echo     PCbuild\rt.bat -q -d -x64 -uall -rwW
-@echo or use `test.bat` in this directory and pass `-x64` as an argument.
-call "%~dp0test.bat" -x64 %*