Revert "build: use the correct variable"

This reverts commit b5a8abd57f23e2f621d5ceb0f64f1bb8f9579c3f.  This
should not be needed as the lldb-server tool will add
`LLDB_CAN_USE_LLDB_SERVER` which will never be set to true on Windows.

llvm-svn: 360745