Remove modules in favour of sympy.physics._biomechanics`

[brocksam]

With https://github.com/sympy/sympy/pull/25567 we will have reached the point where sympy.physics._biomechanics has implemented the functionality of the following modules:

• https://github.com/brocksam/sympy-biomechanics/blob/main/biomechanics/activation.py
• https://github.com/brocksam/sympy-biomechanics/blob/main/biomechanics/curve.py
• https://github.com/brocksam/sympy-biomechanics/blob/main/biomechanics/mixin.py
• https://github.com/brocksam/sympy-biomechanics/blob/main/biomechanics/musculotendon.py

We should remove these from this repo, and update all example scripts to use the implementations directly from SymPy to avoid having duplicate implementations.