Bug 18687: Translatability: abbr tag should not contain lang attribute

Bug 18687: Translatability: abbr tag should not contain lang attribute

In manage-marc-import.tt, we have an abbreviation:

<abbr title="Differences between the original biblio and the imported" lang="en">Diff</abbr>

In translations (e.g. German), the line appears as follows:
<abbr title="Unterschiede zwischen Originaltitelsatz und importiertem Titelsatz" lang="en">Diff</abbr>

The lang attribute is wrong here, it is still "en".
The text language is the same as defined at the top of the page - or with other
words, the lang tag is superfluous.

This patch removes it.

To test:
Verify that code change makes sense.

Passes QA test and the change is logical
Signed-off-by: Alex Buckley <alexbuckley@catalyst.net.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>