Removing unneeded msstr from en_US po file

http://www.collectiveaccess.org/support/forum#/discussion/293090/translation-using-transifex

"It's quite weird : those strings were inside first github version of en_US.po.
You can see it at :
https://github.com/collectiveaccess/providence/blob/68e5709dc2ef83ccbd88c2d30ad160fa47b6c825/app/locale/en_US/messages.po

l.345 and next :
#: c:\providence/app/controllers/find/SearchEntitiesAdvancedController.php:73
#, fuzzy
msgid "displayname"
msgstr "User name"

#: c:\providence/app/controllers/find/SearchEntitiesController.php:71
msgid "surname, forename"
msgstr ""

#: c:\providence/app/controllers/find/SearchEntitiesController.php:72
#, fuzzy
msgid "forename"
msgstr "User name"

I'm removing those weird translations from en_US.po."

Signed-off-by: gautiermichelin <gm@ideesculture.com>