stackrc: Remove USE_PYTHON3

This is no longer necessary and any users of this should be updated to
remove references.

Change-Id: Ice5083d8897376fd2ed6bd509419526e15baaf12
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

Author: stephenfin

stackrc

@@ -126,10 +126,6 @@ if [[ -r $RC_DIR/.localrc.password ]]; then
     source $RC_DIR/.localrc.password
 fi
 
-# Control whether Python 3 should be used at all.
-# TODO(frickler): Drop this when all consumers are fixed
-export USE_PYTHON3=True
-
 # Adding the specific version of Python 3 to this variable will install
 # the app using that version of the interpreter instead of just 3.
 _DEFAULT_PYTHON3_VERSION="$(_get_python_version python3)"