Remove `try --explain EXXXX` if we know that an explanation is not written

[nagisa]

Ideally we would have explanations written for every error code, but that is a long term effort and we should stop giving people false hope about things possibly existing when we know that they do not exist in cases like these:

error: aborting due to previous error

For more information about this error, try `rustc --explain E0495`.

$ rustc --explain E0495
error: no extended information for E0495