[#212] Remove set voting period

[sras]

Description

Problem: We have to remove 'set_voting_period' entrypoint from the contract interface.

Solution: Remove the entrypoint, and update storage to remove the 'min_voting_period' and 'max_voting_period' values from configuration. Also moves 'voting_period' configuration to the config section of storage. The Makefile is also updated to remove reference to voting_period limits. Also updates documentation.

Related issue(s)

Resolves #212

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