exp branch: Add a flag --remove or --delete to remove the persisted experiment

[iesahin]

Current behavior

dvc exp branch exp-123ab my-exp

creates a new Git branch my-exp.

dvc exp list still lists the branched experiment though.

dvc exp list
main:
    exp-123ab

Proposed behavior

A flag in dvc exp branch (and maybe dvc exp apply) to delete the experiment record from DVC once they are promoted to a branch.

dvc exp branch --remove exp-123ab my-exp
dvc exp list 
main:
   (no exp-123ab)    

This might also be the default behavior that can be turned off with a --no-remove flag.

[daavoo]

Closing in favor of #7576