Bug 6733: change branchcode call in Labels to use userenv instead of LoginBranchname

Bug 6733: change branchcode call in Labels to use userenv instead of LoginBranchname

In the 5 places in the label editor, the user's current branch is determined by taking
the LoginBranchname and doing a reverse look up for the branchcode.  This is an unnecessary
query, since C4::Context->userenv has the immediate access to the branchcode; this is how
it's summoned in all other points in the Koha code.

This addresses bug 6673 because, since the move to T:T, the call in tools/manage-marc-import.pl
has used in correct syntax:  $template->param('LoginBranchname') instead of $template->{VARS}->param('LoginBranchname')

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Ian Walls <ian.walls@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>