Consider removing openzeppelin-contracts dependency

[cam-schultz]

Context and scope
With #397, we added support for upgradeable variants of Teleporter utilities to allow for Teleporter apps to be implemented in an upgradeable fashion. In doing so, we added an additional submodule dependency for openzeppelin-contracts-upgradeable in addition to the existing openzeppelin-contracts submodule.

openzeppelin-contracts-upgradeable itself includes openzeppelin-contracts as a submodule, so our openzeppelin-contracts submodule is likely redundant.

Discussion and alternatives
This may help reduce the chance of import conflicts, which can cause headaches in CI.

Open questions
We currently don't enforce similar versions for openzeppelin-contracts and openzeppelin-contracts-upgradeable. Are we comfortable using the openzeppelin-contracts version specified in openzeppelin-contracts-upgradeable?

[geoff-vball]

Completed