The pit can not be dispelled, since it is not magical in any sense, and it is permanent.
The text of Dispel Magic spell says (emphasis mine):
Choose one creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends.
To check if something is magical, one should apply the method described in the Sage Advice Compendium (see the question Is the breath weapon of a dragon magical?). The SAC says that if the answer to at least one of the following question is "yes", then this something is magical:
- Is it a magic item?
- Is it a spell? Or does it let you create the effects of a spell
that’s mentioned in its description?
- Is it a spell attack?
- Is it fueled by the use of spell slots?
- Does its description say it’s magical?
Let's check these possibilities one by one.
- No, the pit is not a magic item, it is just a hole in the ground. Being very pedantic, there's no magic item named Pit in any manual.
- It is not a spell, and it is not the effect of any particular spell. If it were so, the description of the Robe of Useful Items would have said so.
- No: no spell slot have been used to create it.
- The description says simply
Pit (a cube 10 feet on a side), which you can place on the ground within 10 feet of you
and nowhere it says that it is a magical pit.
The pit created by the patch of the Robe of Useful item does not satisfy any requirement to be considered magical, hence it can not be dispelled.
Moreover, the description does not have any duration entry: the pit is placed forever, or at least until someone fill it.