Merge branch 'rl/remote-allow-missing-branch-name-merge'

"git remote rm X", when a branch has remote X configured as the
value of its branch.*.remote, tried to remove branch.*.remote and
branch.*.merge and failed if either is unset.

* rl/remote-allow-missing-branch-name-merge:
remote: ignore failure to remove missing branch.<name>.merge

diff --cc builtin/remote.c
Simple merge

diff --cc t/t5505-remote.sh
Simple merge