Drop unused shebangs from repo test fixture scripts

[musicinmybrain]

These are not really doing any harm, but they are not doing any good either, and are perhaps slightly misleading:

• Since the scripts do not have executable filesystem permissions, the shebang (#!) line is not used
• For non-executable scripts like this, gix-testtool always executes them with bash: https://github.com/Byron/gitoxide/blob/a807dd1ffb05efd177700d065095249e6c4b3c68/tests/tools/src/lib.rs#L555-L560

An alternative would be to chmod a+x tests/fixtures/*.sh so that the shebang lines can be useful – and are used, if I read the gix-testtools code correctly.

[spenserblack]

Besides running the script, shebangs can be useful as hints for language detection (editors, GitHub stats). But it shouldn't matter since they all have .sh extensions.