Disable lock screen on iPhone
Apple didn't program a way to eliminate the “Slide to unlock” screen. Without jailbreaking the phone, you can't get around this preventative feature.

(I guess the rationale behind this design is as a precaution against accidentally doing something unwanted while your phone sits in your pocket/purse/hand and the power button gets bumped. This also is how the lock screen is designed to work when the device is not powered off.)

The best you can do with a stock OS is to disable auto-locking in “Settings > General > Auto-Lock > Never”. However, whenever the phone is locked manually you will have to slide to unlock before using the device.