CI: don't set BOOST_ROOT to an empty string

This was initially added in eb76ba7031 to avoid picking
it up from azure/gha images.

Looks like it is no longer set for some time now though:
https://github.com/actions/runner-images/pull/540
so should no longer be needed.

The empty string makes some meson tests fail which expect a path.
A recent msys2-runtime change now lets empty env vars through to child processes
whereas before they would get filtered out, which is why this worked before.

.github/workflows/msys2.yml

@@ -106,7 +106,6 @@ jobs:
             # Make sure it is on the PATH
             pypy3 -c "import sys; print(sys.version)"
           fi
-          export BOOST_ROOT=
           export PATHEXT="$PATHEXT;.py"
 
           if [[ '${{ matrix.COMPILER }}' == 'clang' ]]; then