Use PyBaMM's create_gif function and remove BattBot's create_gif function

[Saransh-cpp]

Now that #1754 is merged, the bot can do something like this -

batch_study.create_gif()

instead of -

BattBot/bot/plotting/comparison_generator.py

Line 144 in 5f5d68c

create_gif(batch_study)

This can also be changed here -

BattBot/bot/plotting/comparison_generator.py

Line 227 in 5f5d68c

create_gif(batch_study, labels=labels)

The create_gif function and the tests can also be removed.