t5526: drop the prereq expecting the default branch name `main`

t5526: drop the prereq expecting the default branch name `main`

Initially, we started converting this test script in anticipation for
renaming the default branch name to `main`. To that end, we partially
converted it to accommodate for that default branch name, marking the
now-failing test cases with a prereq that was designed to be fulfilled
once the rename was complete.

However, the effort to move to the branch name `main` needs quite a bit
longer, as it was decided that we need a deprecation phase first.

To avoid keeping t5526 in limbo for such a long time, we just made it
independent of the actual default branch name used by Git. Therefore,
that prereq is no longer necessary, and we can drop it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>