Remove `opscenter` target from OSS Makefile

[eldios]

We should remove the opscenter (and related) target from OSS Makefile since that's not functional anymore since when we moved this part only in the Enterprise edition.

[kontsevoy]

@eldios the OSS edition Gravity does have some useless targets on purpose, they simplify enterprise builds so we don't have to update existing automation in the process of open sourcing.

[kontsevoy]

if I answered your question, do you mind closing this pls? thx

[eldios]

Had chat about this with @kontsevoy and decided that we should probably move the extra targets into a internal.mk file which we can include at the bottom of Makefile with a comment saying "for internal use only" or similar explanatory comment.