It is a bit of a pain that any entries on that screen can only be re-allocated - not deleted. If you go to the 'users' folder in your game, find the 'settings.ini' file (or files, if you have more than one pilot) you can manually delete the entry from there. You will find it in the [HotKey pilot] section of the file.
Do I need to say make a copy first?
[HotKey pilot]
Shift Z=AIRCRAFT_TRIM_R_0
Shift X=AIRCRAFT_TRIM_R_0
Shift W=EngineSelect2
Shift T=AIRCRAFT_TAILWHEELLOCK
Shift S=AIRCRAFT_BRAKESHOE
Shift R=EngineSelect4
Shift Q=EngineSelect1
Insert=Brake_Left
Joystick11 JoystickDevice0=Weapon Salvo Size
Shift F=EngineExtinguisher
Shift E=EngineSelect3
Shift C=Open/Close Side Hatch
Joystick5 JoystickDevice0=Weapon2
Ctrl Z=AIRCRAFT_TRIM_R_MINUS
Ctrl X=AIRCRAFT_TRIM_R_PLUS
Ctrl W=WINGFOLD
Shift 4=Step90
Shift 3=Step60
Ctrl T=AIRCRAFT_DROP_TANKS
Shift 2=Step30
Ctrl S=AIRCRAFT_DOCK_UNDOCK
Shift 1=Step0
Delete=Brake_Right
Ctrl M=Mix100
F10=Open/Close Bay Doors
Ctrl H=AIRCRAFT_CARRIERHOOK
Shift Up=AIRCRAFT_TRIM_V_0
Shift Left=AIRCRAFT_TRIM_H_0
Underscore=AIRCRAFT_POWER_MINUS_5
Ctrl C=COCKPITDOOR
NumPad-=StepMinus5
Joystick0 JoystickDevice0=Weapon0
NumPad+=StepPlus5
NumPad-9=SIGHT_SPD_PLUS
NumPad-8=SIGHT_ALT_PLUS
NumPad-7=SIGHT_DIST_PLUS
NumPad-6=SIGHT_SIDE_LEFT
NumPad-5=SIGHT_AUTO_ONOFF
NumPad-4=SIGHT_SIDE_RIGHT
NumPad-3=SIGHT_SPD_MINUS
NumPad-2=SIGHT_ALT_MINUS
NumPad-1=SIGHT_DIST_MINUS
BackSlash=RADAR_MODE_TOGGLE
Z=RudderLeft
Ctrl Down=AIRCRAFT_TRIM_V_PLUS
X=RudderRight
Ctrl Right=AIRCRAFT_TRIM_H_PLUS
W=Boost
Ctrl Up=AIRCRAFT_TRIM_V_MINUS
V=AIRCRAFT_FLAPS_NOTCH_DOWN
Ctrl Left=AIRCRAFT_TRIM_H_MINUS
R=Radiator
Joystick7 JoystickDevice0=BOMB_RELEASE_MODE
N=AIRCRAFT_STABILIZER
Joystick1 JoystickDevice0=Weapon3
I=AIRCRAFT_TOGGLE_ENGINE
G=Gear
F=AIRCRAFT_FLAPS_NOTCH_UP
Shift BackQuote=StepAuto
B=Brake
Equals=AIRCRAFT_POWER_PLUS_5
9=Power90
8=Power80
7=Power70
Alt Right=EngineToggleRight
6=Power60
Alt Up=EngineToggleAll
5=Power50
Alt Left=EngineToggleLeft
4=Power40
3=Power30
2=MagnetoPlus
0=Power100
Period=AIRCRAFT_RUDDER_LEFT_1
Comma=AIRCRAFT_RUDDER_CENTRE
Joystick2 JoystickDevice0=Weapon1
Down=ElevatorDown
Right=EngineSelectRight
Up=EngineSelectAll
Left=EngineSelectLeft
PageDown=AIRCRAFT_GEAR_DOWN_MANUAL
PageUp=AIRCRAFT_GEAR_UP_MANUAL
Space=Deploy Drag Chute
Alt=DeployChaff
Joystick9 JoystickDevice0=ROCKET_SELECT
Joystick3 JoystickDevice0=AIRCRAFT_TOGGLE_AIRBRAKE
Enter=DeployFlare
Backspace=Stabilizer
Which part is that?