Remove libmamba installation

Newer versions of conda come with libmamba. Installing libmamba
is forcing conda to be an older version. Just remove the
installations of libmamba.

Signed-off-by: Patrick Avery <patrick.avery@kitware.com>

Author: psavery

packaging/environment.yml

@@ -10,4 +10,3 @@ dependencies:
   - conda-build
   - conda-pack
   - click
-  - conda-libmamba-solver