Formal:
* Superfluous
* Redundant
* Extraneous
* Inessential
* Nonessential
* Supererogatory (implies doing more than is required)
Informal:
* Unneeded
* Needless
* Pointless
* Unwanted
* Excessive
* Overkill
* Frivolous
* Wasteful
The best word to use will depend on the context and the tone you want to convey.