git: Unconditionally ignore gitconfig files

A while ago we have introduced the ignore_gitconfig configuration. If set, we will override GIT_CONFIG_SYSTEM and GIT_CONFIG_GLOBAL as well as override XDG_CONFIG_HOME so that Git won't pick up gitconfig files found in any of these scopes. The goal of this is that we only ever use the Git configuration that is found either in Gitaly's config.toml or in the repository-local gitconfig.

This toggle has been enabled in all distributions unconditionally already and was scheduled for removal in v16.0. So let's remove that toggle and unconditionally ignore any global- or system-level gitconfig files.

Closes #5073 (closed).