Remove multiple commit prefixes

[serhiy-storchaka]

Usually backports are created directly from the main branch. But if there were significant changes in the previous version, it is simpler to create backports to older versions from this version. So you can get titles like "[3.11] [3.12] Fix something..."

This is redundant, only the leftmost prefix matters. It increases the length of the PR title (limited by GitHub UI) and the first line in the commit message (limited by the width of the screen or whatever). The way of creating backports is irrelevant, it is just what was more convenient for the author.