fix(optimistic-rewarder): [N07] addressed unused imports

[chrismaree]

Motivation
OZ identified the following issue:

To improve readability of the code, consider removing the following unused imports:

• The OptimisticRewarderBase contract imports the unused Constants contract
• The OptimisticRewarderToken contract imports unused OptimisticRewarder contract
• The OptimisticRewarderCreator contract imports unused ERC721 and Testable contracts
• the OptimisticStaker contract imports usused OptimisticRewarder and
  OptimisticRewarderToken contracts

This PR addresses this by removing these imports.

Testing

Check a box to describe how you tested these changes and list the steps for reviewers to test.

• Ran end-to-end test, running the code as in production
• New unit tests created
• Existing tests adequate, no new tests required
• All existing tests pass
• Untested

[chrismaree]

Note that OZ commented that the The OptimisticRewarderBase contract imports the unused Constants contract which is redundant. this is not true. this contract requires this for accessing the OracleInterfaces library to pull things like OracleInterfaces.Store