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.

Author: bors

.gitignore

@@ -95,3 +95,4 @@ src/etc/dl
 build/
 i686-pc-mingw32/
 src/librustc/lib/llvmdeps.rs
+*.pot