Use default solver (libmamba) when building conda package (#2562)

Summary:
## Motivation

[This issue](conda/conda#13412) should be resolved now that libmamba 2.0.0 is out.

### Have you read the [Contributing Guidelines on pull requests](https://github.com/pytorch/botorch/blob/main/CONTRIBUTING.md#pull-requests)?

Yes

Pull Request resolved: #2562

Test Plan: Run nightly cron and make sure conda package build succeeds

Reviewed By: Balandat

Differential Revision: D63789161

Pulled By: esantorella

fbshipit-source-id: 5229c86dbbb3e8fe2e5c8e96cde42b077d6a5088

.github/workflows/deploy_on_release.yml

@@ -60,8 +60,6 @@ jobs:
     - name: Install dependencies
       shell: bash -l {0}
       run: |
-        # avoid conda bug in >=23.10.0: https://github.com/conda/conda/issues/13412
-        conda config --set solver classic
         conda install -y setuptools_scm conda-build conda-verify anaconda-client
         conda install -y scipy sphinx pytest flake8 multipledispatch
         conda install -y -c pytorch pytorch cpuonly

.github/workflows/nightly.yml

@@ -79,8 +79,6 @@ jobs:
       # Don't need most deps for conda build, but need them for testing
       # We do need setuptools_scm though to properly parse the version
       run: |
-        # avoid conda bug in >=23.10.0: https://github.com/conda/conda/issues/13412
-        conda config --set solver classic
         conda install -y scipy multipledispatch setuptools_scm conda-build conda-verify
         conda config --set anaconda_upload no
         conda install -y -c pytorch-nightly pytorch cpuonly

.github/workflows/test.yml

@@ -36,8 +36,6 @@ jobs:
       env:
         ALLOW_LATEST_GPYTORCH_LINOP: true
       run: |
-        # avoid conda bug in >=23.10.0: https://github.com/conda/conda/issues/13412
-        conda config --set solver classic
         conda install pytorch torchvision -c pytorch
         conda install -y pip scipy sphinx pytest flake8
         pip install git+https://github.com/cornellius-gp/linear_operator.git

.github/workflows/test_stable.yml

@@ -31,8 +31,6 @@ jobs:
     - name: Install dependencies
       shell: bash -l {0}
       run: |
-        # avoid conda bug in >=23.10.0: https://github.com/conda/conda/issues/13412
-        conda config --set solver classic
         conda install -y -c pytorch pytorch cpuonly
         conda install -y pip scipy pytest
         conda install -y -c gpytorch gpytorch