Update doc/po4a.conf for recent changes.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Remove *.pot files and ignore them now.

The .pot files can be generated automatically and the files contain
timestamps in their content. They can cause huge conflicts and take huge
space even if you are not a translator.

This commit is a part of improvement discussed on
https://github.com/mozilla/rust/pull/11383 .

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
auto merge of #11427 : omasanori/rust/remove-pot, r=alexcrichton

`doc/po/*.pot` files are generated automatically using `po4a` and they includes timestamps in their content. The files can cause huge conflicts unnecessarily. Also, removing them save disk space if you are not a translator.