Remove mamba installation instructions

[leehach]

As of conda 23.10, libmamba is the default environment solver (https://docs.conda.io/projects/conda/en/latest/release-notes.html#version-23.10.0). Therefore in Section 1.7, I recommend removing the instructions to install mamba, and replace mamba env create -f environment.yml with conda env create -f environment.yml.

Of course, the instructions you have work fine as is (in fact, the exact same thing is happening under the hood), but I think it beginners will not understand the whys of the current instructions.