Removing unneeded msstr from en_US po file

http://www.collectiveaccess.org/support/forum#/discussion/293090/translation-using-transifex

See https://github.com/collectiveaccess/providence/pull/48 for providence pull request.

Signed-off-by: gautiermichelin <gm@ideesculture.com>
Merge pull request #7 from ideesculture/master

Removing unneeded msstr from en_US po file