Remove `plugin` argument of `io.imsave` to suppress warning logs

**Is your feature request related to a problem? Please describe.**

Whenever `image_utils.save_image` is called, https://github.com/angelolab/alpineer/blob/main/src/alpineer/image_utils.py#L27 will throw a warning highlighting how `plugin` will be deprecated (we currently set it to `"tifffile"`).

This causes an issue when a function makes multiple calls to it (ex. iterating through each FOV on a run, and writing out each channel tiff using this function). This will throw a ton of warning messages to the user which can be confusing and indecipherable.

**Describe the solution you'd like**

We need to remove this `plugin` param. If it's absolutely necessary, we should find a way to replicate what it currently does.