Remove the user warning for mismatches in prefixes and the python path.

When `enpkg --prefix` is run, we can safely assume that the user knows
what he/she is trying to do.  The warnigns don't make sense, for
example, when the user is installing packages into a virtual
environment based off of the python environment in which this
enstaller code is running.  The check makes sense, only if the user is
installing packages from a child virtual environment into a parent
python environment.  I think it is safe to assume that the user knows
what they are doing here, and the warning is confusing to most other
users.

When enstaller is being used as a library, to install packages into
prefixes separate from the one where the code is running, the warnings
are meaningless.