[ISSUE-697] Cleanup the Serialization support

[d-kamath]

Closes #697

Description

This PR includes several updates for serialization cleanup and improvements to the Serializer class:

1. Removed code previously used to create synapses from synapse weights saved in the serialization file: Connections::createSynapsesFromWeights. This is no longer needed as we serialize all values including synapses.
2. Updated comments and functions in the Serializer class for better clarity.
3. Removed unused variables: totalNeurons_ and numCallerVertices_.
4. Removed unused functions: Simulator::copyGPUSynapseToCPU and Simulator::copyCPUSynapseToGPU. These functions are moved into Model class.

Checklist (Mandatory for new features)

• Added Documentation
• Added Unit Tests

Testing (Mandatory for all changes)

• GPU Test: test-medium-connected.xml Passed
• GPU Test: test-large-long.xml Passed