[python] Fix varobj crash if no python is installed

[Tom Tromey]

>>>>> "Jan" == Jan Kratochvil <jan.kratochvil@redhat.com> writes:

Jan> Reproducible and uncommitted on [archer-tromey-python].

If we're keeping archer-tromey-python as the topic branch, we ought to
double-commit such changes.

Jan> 	Fix varobj crash if no python is installed.
Jan> 	* varobj.c (install_default_visualizer [!HAVE_PYTHON]): Remove the
Jan> 	error call.

Thanks.

Tom