Which reagent can I use to remove carbon from oven?
As a disclaimer, this is for information only.  Anyone using this information assumes all associated risks.  The manufacturer's label should be followed for any cleaning products.

The exact chemical composition of the goo is not precisely known. It is likely either tar or creosote existing as some form of hydrocarbons.  These hydrocarbons are tangled up to the point it is extremely difficult to dissolve.  The temperature required to burn this material is likely too high for anything short of a ultra high temperature industrial furnace.  

A good way to clean it is to use oven cleaners which take advantage of the saponification reaction. Saponification will loosen the material; however, some scrubbing and/or scraping will still be involved.

For safety when oven cleaning be sure to have good ventilation.  Wear gloves, eye and face protection, as well as not having skin exposed. Caustic cleaning compounds and vapors from the cleaning are hazardous.

Most oven cleaners have multiple components. 


A caustic solution causes a saponification reaction.  The saponification reaction is commonly used to make soap from saponifiable oils.  Because the oils in the material to be cleaned are likely saponifiable,  the saponification reaction makes the material much more likely to dissolve in water.  
A foaming component creates a foam. When foam bubbles pop, the popping action works to break up the tangled bonds.
A compound may be included to ensure the entire solution will stick to a non-horizontal surface.
Another compound may be included to seal the solution so as to prevent oxidation.  Oxidation may interfere with the saponification.


The trick to getting oven cleaners to be effective, in my experience, is in the timing.  Timing is important because the contents of tars are not exactly known, so the softening reaction is brief.      


A warm surface is easier to clean; however, a hot surface will dry the cleaner before it has a chance to work. 
The oven should not be powered when attempting any cleaning.  
When any oven cleaner is first applied, it is important to leave the cleaner alone and give it time to work. Ten minutes is usually sufficient.  
Once dissolution occurs there is a brief window when the tar or creosote can be easily removed with a nylon brush or sometimes even a paper towel.
When the materials dissolved, vapors which were previously trapped in the tars may be released.  These released vapors are not good to breathe.  
Sometimes it is necessary to get deeper layers of tar.  The cleaner can be reapplied up to three times before the solution dries if necessary.  Each time allow it to go undisturbed for ten minutes.  (In my experience, by the fourth application, cleaning the tar is simply not worth the effort.) 
If too much time passes and the solution dries, then the tar is more difficult to clean.  The caustic solution can also damage metal components.
Rinse the oven of all cleaner when done.  Caustic residues can damage the oven if left on the surface.