build-hafnium.sh: remote out/reference folder during clean

The $PATH environment variable is used to set the path to the Clang
toolchain while building the Hafnium project. This toolchain path is
used to set the $toolchain_lib variable, which is saved in the
out/reference/args.gn file. When the Clang toolchain path is changed,
the usual practice is to perform a clean build of Hafnium and update the
$PATH environment variable to the new Clang path before rebuilding.
However, the clean command is not effectively removing the outdated
arg.gn file in the out/reference folder, leading to build failures due
to the use of an old $toolchain_lib variable. To avoid this issue, we
delete the out/reference folder altogether, ensuring that the
$toolchain_lib variable is always up to date.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I346d87f6243c993e80aba99849141884cff0faca