There is little point in using the LWLockRegisterTranche mechanism for
built-in tranche names.  It wastes cycles, it creates opportunities for
bugs (since failing to register a tranche name is a very hard-to-detect
problem), and the lack of any centralized list of names encourages
sloppy nonconformity in name choices.  Moreover, since we have a
centralized list of the tranches anyway in enum BuiltinTrancheIds, we're
certainly not buying any flexibility in return for these disadvantages.

Hence, nuke all the backend-internal LWLockRegisterTranche calls,
and instead provide a const array of the builtin tranche names.

(I have in mind to change a bunch of these names shortly, but this
patch is just about getting them into one place.)

Discussion: https://postgr.es/m/9056.1589419765@sss.pgh.pa.us