Redundant ModeIcon class

[binh-dam-ibigroup]

Is this duplicative of https://github.com/opentripplanner/otp-ui/blob/master/packages/icons/src/trimet-mode-icon.js? I suppose that's ok if so, but we may want to create an issue for this repo identifying potential spots for refactoring. This could get messy if we have a bunch of the same code repeated in different packages.

Originally posted by @landonreed in #20

[binh-dam-ibigroup]

@landonreed How should we handle this? Get rid of ModeIcon and update TrimetModeIcon accordingly?