simplify model copy

[valentinsulzer]

Description

Remove model.new_empty_copy() and simplify model.new_copy().
Not sure why I made it so complicated in the first place, but this seems to work fine and be faster

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

Merging #1977 (bfcb2c6) into develop (fa337e4) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1977      +/-   ##
===========================================
- Coverage    99.32%   99.32%   -0.01%     
===========================================
  Files          346      346              
  Lines        19038    19013      -25     
===========================================
- Hits         18910    18885      -25     
  Misses         128      128              

Impacted Files	Coverage Δ
...m/models/full_battery_models/base_battery_model.py	99.76% <ø> (-0.01%)	⬇️
...models/full_battery_models/lead_acid/basic_full.py	100.00% <ø> (ø)
...odels/full_battery_models/lithium_ion/basic_dfn.py	100.00% <ø> (ø)
...odels/full_battery_models/lithium_ion/basic_spm.py	100.00% <ø> (ø)
...tery_models/lithium_ion/electrode_soh_half_cell.py	100.00% <ø> (ø)
pybamm/simulation.py	98.25% <ø> (ø)
pybamm/discretisations/discretisation.py	99.79% <100.00%> (ø)
...bamm/expression_tree/operations/replace_symbols.py	100.00% <100.00%> (ø)
pybamm/models/base_model.py	99.23% <100.00%> (-0.02%)	⬇️
pybamm/parameters/parameter_values.py	99.46% <100.00%> (ø)

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[brosaplanella]

Looks great, thanks Tino!

[brosaplanella]

What was the point of these tests, originally?

[valentinsulzer]

just coverage