add back add/remove_qubit to density matrix sim state

[sanuj]

For #6081.

add/remove_qubit() was added in #6108 and removed in #6127. I've added back the functions and updated remove_qubit() to self.factor() inplace to make the test (simulation_state_test.test_delegating_gate_channel()) pass.

The test was failing because the final density matrices were different for the control and test circuit. The simulate(test_circuit).final_density_matrix.shape was (2, 2, 2, 2) while simulate(control_circuit).final_density_matrix.shape was (2, 2). My guess was that the adding qubit was working fine but remove_qubit() was not able to remove the qubit. After debugging, I found that remove qubit was also working fine but self.factor() was not being done inplace and the density matrix was not getting updated during the removal of the qubit.

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[tanujkhattar]

This is great, thanks Sanuj for debugging the issue and sending the fix! Good to know that it was a minor fix and not a bigger issue with rest of the infrastructure.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (ed26d2f) 97.60% compared to head (0764095) 97.60%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6259      +/-   ##
==========================================
- Coverage   97.60%   97.60%   -0.01%     
==========================================
  Files        1116     1116              
  Lines       95848    95858      +10     
==========================================
+ Hits        93556    93564       +8     
- Misses       2292     2294       +2     

Files Changed	Coverage Δ
...q-core/cirq/sim/density_matrix_simulation_state.py	99.02% <100.00%> (+0.10%)	⬆️
cirq-core/cirq/sim/simulation_state_test.py	98.51% <100.00%> (ø)

... and 2 files with indirect coverage changes

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