Remove assertions that some compiler say are tautological

Remove assertions that some compiler say are tautological

To avoid the compiler warnings:

    launch_backend.c:211:39: warning: comparison of constant 16 with expression of type 'BackendType' (aka 'enum BackendType') is always true [-Wtautological-constant-out-of-range-compare]
    launch_backend.c:233:39: warning: comparison of constant 16 with expression of type 'BackendType' (aka 'enum BackendType') is always true [-Wtautological-constant-out-of-range-compare]

The point of the assertions was to fail more explicitly if someone
adds a new BackendType to the end of the enum, but forgets to add it
to the child_process_kinds array. It was a pretty weak assertion to
begin with, because it wouldn't catch if you added a new BackendType
in the middle of the enum. So let's just remove it.

Per buildfarm member ayu and a few others, spotted by Tom Lane.

Discussion: https://www.postgresql.org/message-id/4119680.1710913067@sss.pgh.pa.us