WIP: Remove the pip build element & source plugins
Description
WIP until the plugins has been merged to bst-plugins-experimental, at which point BST_PLUGINS_EXPERIMENTAL_VERSION
in tox.ini can be bumped as well as BST_EXT_REF
in gitlabci which has two references. This is part of #1224
MR for bst-plugins-experimental: bst-plugins-experimental!78
Changes proposed in this merge request:
- Remove pip build element & source plugins with related tests
- Update NEWS entry for breaking change
Edited by Javier Jardón