Bug 29747: Delete columns that don't exist from settings

Bug 29747: Delete columns that don't exist from settings

This patch simply checks if we are callinmg from the plugin, and removes
two columns from the settings if so

To test:
 1 - In Admin->Marc bibliographic framework got to Marc Structure for default framework
 2 - Search for 856$u
 3 - Set Plugin to upload.pl
 4 - Edit a record in the default framework
 5 - Under 856u, click 'Upload'
 6 - Uplaod a file and then click 'Choose'
 7 - nothing happens
 8 - View consiole (f12) and see error
 9 - Apply patch
10 - Reload the page
11 - No more error
12 - Click choose
13 - Record link is populated

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>