Fix et-al in ecology-letters bibliography section

The "et al" phrase was being printed for all references, even those with
all authors printed. To fix this I have removed a label line and added a
necessary prefix space between the authors and the "et al". Besides
forcing "et al" to appear for all references, I'm not sure if there was
another purpose to the label line. Perhaps there is a better fix.
Remove citation-number block from ecology-letters

The author instructions specify that the bibliography should be:
"listed in alphabetical order at the end of the paper, and numbered
sequentially (in the reference list only). A number and a line break
should be added between each reference."
http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1461-0248/homepage/ForAuthors.html

The numbers that were added were the `citation-numbers`, which
corresponded to the order cited, not the alphabetical order. It may not
be possible to add this type of numbering in CSL. Further, if these
numbers can be added, there needs to be a line break after the number.
Therefore, perhaps it is better if the number is not added at all and
the user can add these as a last-minute manual edit before paper
submission.

The official Ecology Letters format (as published) omits these numbers.