t7506: clean up .gitmodules properly before setting up new scenario

t7506: clean up .gitmodules properly before setting up new scenario

In t/t7506-status-submodule.sh at some point a new scenario is set up to
test different things, in particular new submodules are added which are
meant to completely replace the previous ones.

However before calling the "git submodule add" commands for the new
layout, the .gitmodules file is removed only from the working tree still
leaving the previous content in current branch.

This can break if, in the future, "git submodule add" starts
differentiating between the following two cases:

  - .gitmodules is not in the working tree but it is in the current
    branch (it may not be safe to add new submodules in this case);

  - .gitmodules is neither in the working tree nor anywhere in the
    current branch (it is safe to add new submodules).

Since the test intends to get rid of .gitmodules anyways, let's
completely remove it from the current branch, to actually start afresh
in the new scenario.

This is more future-proof and does not break current tests.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Signed-off-by: Junio C Hamano <gitster@pobox.com>