Duplicate error code

[Trott]

ERR_UNKNOWN_BUILTIN_MODULE is listed in the legacy section of the docs as added in 8.0.0 and removed in 9.0.0.... and also listed in the current section.

[Trott]

Since the entries are (nearly) identical and we are using the error code, I'll delete the one in the legacy section.

[Trott]

#35031