Delete main branch and rename develop to main branch

[ypid]

From #550 (comment), I thought about it and I would also argue that maintaining two branches provides no real benefit.The only benefit I see is if a hot fix release needs to be made. But in that rare case, a git branch could be created based on the last release commit and the release could be made in this branch. Then it could be merged back into main. Test #553

Introduced in: #497

@KristjanESPERANTO

[KristjanESPERANTO]

I think the two-branch approach made sense when I didn't have write access to the main branch yet. Now that I have it and can accept PRs, I think the additional branch is unnecessary.

[ypid]

Should be all done.