Stop mirroring of main branch in master

[psaavedra]

The rationale behind this delayed task is to give time enough to third-parties to make a soft transition to update their references from master to main giving 1-year period of grace-time. Therefore this issue must be done not before 2022-02-10.

Required tasks:

• Revert the sync action added in 05a0629
• Remove the master branch (alternatively just let it die alone like an abandoned branch).

Related with: #219