Remove efficientnet-pytorch dependency

[adamjstewart]

I first mentioned this in #918, but we should really remove efficientnet-pytorch for the following reasons:

• It is no longer receiving bug fixes (last commit is from over 4 years ago)
• It does not have wheels on PyPI (makes installation challenging due to missing setuptools dependency)
• All of the models it provides are already available in the well-maintained timm library
• Fewer lines of code in SMP that we need to maintain without fewer features

If we need to ensure backwards compatibility, we can add aliases to the timm implementations.

P.S. I'm also planning on doing this for as many of our other dependencies as I can. I'll wait to see if this PR gets any traction before starting on pretrainedmodels next.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
segmentation_models_pytorch/encoders/__init__.py	85.88% <ø> (-0.33%)	⬇️

... and 1 file with indirect coverage changes