slither-flat: add --remove-assert flag

[montyly]

slither-flat --remove-assert file.sol will export the flattened code where the call to assert() are being commented.

A typical use case is the write of properties to be tested with echidna/manticore, that should not be present in the deployed code.

This must be used with precaution. All the assert that are commented are printed by the tool, and should be reviewed manually.

Fix #366