Revert temporary jmid custom repo

[jmid]

Following the merge of shym/custom-opam-repository#2 this little PR reverts the temporary jmid custom repo

[jmid]

CI summary:

• Linux trunk debug timeout during a 11469.1s Lin DSL Out_channel test with Domain Long Out_channel shrinking runs cause timeout #378
• Windows 5.1 timeout during Mash up of threads and domains [ocaml5-issue] Windows failures on threadomain #203
• Cygwin 5.1 part 1 timeout during a 62.4s negative Lin DSL Dynlink test with Domain due to [ocaml5-issue] Deadlock in Dynlink test on Cygwin+MinGW+MSVC #307 or [ocaml5-issue] General Cygwin slowness #384
• Cygwin 5.1 part 2 timeout during a 12331.4s Lin ref int64 test with Thread shrink run Improve reproducibility of Lin/STM Thread modes #338
• Linux s390x 5.2+trunk failed midway with a Disconnected: Switch turned off after a 3588.8s STM Weak test parallel and a 2932.9s STM Weak HashSet test parallel
• Linux ppc64le 5.2+trunk triggered the STM Float Array test sequential crash [ocaml5-issue] Crashes and hangs on ppc64 trunk/5.2 #380

All of these are known and unrelated to the PR, so I'll merge