CI: Remove defaults from config.yml

[mbargull]

This is in conjunction with bioconda/bioconda-common#54 to remove defaults channel from our builds.

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General instructions

• If this PR adds or updates a recipe, use "Add" or "Update" appropriately as the first word in its title.
• New recipes not directly relevant to the biological sciences need to be submitted to the conda-forge channel instead of Bioconda.
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Instructions for avoiding API, ABI, and CLI breakage issues

Conda is able to record and lock (a.k.a. pin) dependency versions used at build time of other recipes.
This way, one can avoid that expectations of a downstream recipe with regards to API, ABI, or CLI are violated by later changes in the recipe.
If not already present in the meta.yaml, make sure to specify run_exports (see here for the rationale and comprehensive explanation).
Add a run_exports section like this:

build:
  run_exports:
    - ...

with ... being one of:

Case	run_exports statement
semantic versioning	{{ pin_subpackage("myrecipe", max_pin="x") }}
semantic versioning (0.x.x)	{{ pin_subpackage("myrecipe", max_pin="x.x") }}
known breakage in minor versions	{{ pin_subpackage("myrecipe", max_pin="x.x") }} (in such a case, please add a note that shortly mentions your evidence for that)
known breakage in patch versions	{{ pin_subpackage("myrecipe", max_pin="x.x.x") }} (in such a case, please add a note that shortly mentions your evidence for that)
calendar versioning	{{ pin_subpackage("myrecipe", max_pin=None) }}

while replacing "myrecipe" with either name if a name|lower variable is defined in your recipe or with the lowercase name of the package in quotes.

Bot commands for PR management

Please use the following BiocondaBot commands:

Everyone has access to the following BiocondaBot commands, which can be given in a comment:

@BiocondaBot please update	Merge the master branch into a PR.
@BiocondaBot please add label	Add the please review & merge label.
@BiocondaBot please fetch artifacts	Post links to CI-built packages/containers. You can use this to test packages locally.

Note that the @BiocondaBot please merge command is now depreciated. Please just squash and merge instead.

Also, the bot watches for comments from non-members that include @bioconda/<team> and will automatically re-post them to notify the addressed <team>.

[mbargull]

N.B.: The workflow(s) needs an overhaul generally so we can remove these
undesired parts.
Meaning, we should either remove the caching overall or at least be more
precise with what's being cached (e.g., separate bioconda-utils'
environment via "pkgs_dirs" and "conda_build.root-dir" .condarc config).