Unnecesary "Custom RPC" in network dropdown menu

**Describe the bug**
I have to admit that this is not bug at all, but rather proposal.
There is a lot of distraction inside dropdown - 4 testnets not used by most of users + "Custom RPC"
Adding new network is not action done on daily basis. Once it is done I will not do it next day, week or even month.
But switching existing networks is done very frequently - that is why I propose to remove this position from dropdown. It is duplication of the unfrequently used feature that is already inside settings.

**Steps to reproduce**

1. Open metamask
2. Open network drop down menu
3. Look at the last position

Start at the network dropdown described in the issue and trace where its menu entries are defined. Confirm how the Custom RPC option is exposed alongside the existing networks, then verify that the dropdown no longer shows it while the settings path for adding a custom network remains available.

Written by the indexing model from the issue text.