refactor: Remove unsupported growthRate for demography.

[b-s-code]

Problem

While reading some age structure code, I encountered an unexpected XML attribute.

This attribute is related to population growth.

For at least 10 years, OpenMalaria has just thrown an exception if this attribute is used with any nonzero value. So the code is not really useful.

Solution

This PR removes schema-level support for the XML attribute and deletes code related to it.

Testing

(macOS CI fails but this is addressed in a separate PR #437)

Nonzero values for the growthRate attribute were not supported before this PR anyway.

Before:

When one added growthRate="0.1" to the node in the example, XML, the output when running OpenMalaria is:
Error: Population growth rate provided.
In: example_scenario.xml

After:

Now, if one adds growthRate="0.1" to the node in the example, XML, the output when running OpenMalaria is:
XSD error: instance document parsing failed
:29:83 error: attribute 'growthRate' is not declared for element 'demography'

Documentation

Wiki docs are already consistent with the behaviour before and after this PR, no update is needed.

https://github.com/SwissTPH/openmalaria/wiki/ModelDemography

The simulations also run with constant population sizes

Notes

I chose to leave a variable called rho instead of just hardcoding a value of 0 everywhere it's used because that seems more readable to me.

[b-s-code]

@acavelan could I please get a review?

@melissapenny @nakul7 FYI