assert.3 - Remove the .Rs and .Re macros to resolving WARNING from mandoc -Tlint

[chrisdavidson]

assert.3 - Remove the .Rs and .Re macros to align with the mandoc -Tlint output.

Removal of the .Rs and .Re macros resolves the warning output (from mandoc) of: WARNING: invalid content in Rs block.

The resulting output does not visually change (confirmed with mantra utility and man utility).