How do I remove unwanted emojis from the keyboard on iPhone?

You cannot delete any emojis. As with all device manufactures the emojis are a set approved by the Unicode Consortium and the devices support the latest emoji standards. Unicode occasionally updates the emojis and those will then be reflected on the device when it adopts that version.

If you are referring to your Frequently Used Emojis page, which is located on the first page, just don’t use those Emojis. If you stop using those, eventually they will be cycled out and replaced by your more frequently used Emojis.

If you are referring to the other Emoji page, those have been set by the Unicode Consortium, the ones in charge of making Emojis. You cannot remove those.

You don't. The emoji font is a protected system font. There's no way to pick and choose which emojis appear.

If you don't like particular ones, ignore them and don't use them. It really is that easy.