Removes section about big MRs

What does this MR do?

As discussed in Retrospective 9.2 (https://docs.google.com/document/d/1nEkM_7Dj4bT21GJy0Ut3By76FZqCfLBmFQNVThmW2TY/edit#) this section should be removed:

Filipa: Frontend has documentation about making smaller MR into a big one, which goes against everything talked here,
should this be removed? 
https://docs.gitlab.com/ce/development/fe_guide/index.html#divide-a-big-feature-into-small-merge-requests 
Douwe: Probably :)

I did not add any further explanation as this is not Frontend specific and there is already a point in here: https://about.gitlab.com/handbook/values/#iteration

We can leave the issue open and iterate what's the best way to do it/if we should add it to the docs or not :)

What are the relevant issue numbers?

#36072 (closed)