Fix error after submitting previously cleared ArboryFile file input

[holystix]

Fixes error where a resource.content.file.remove=1 parameter is needlessly submitted. Error occurs in the following scenario:

1. A file is added and successfully submitted to an ArboryFile form field input.
2. File input is cleared (using X button) without submitting the form or reloading the page.
3. A new file is added to the file input.
4. Form is submitted.

[coveralls]

Coverage remained the same at 2.151% when pulling e83905a on holystix:arbory_file_remove_and_add_fix into a1b7b3d on arbory:master.

[coveralls]

Coverage remained the same at 2.151% when pulling e83905a on holystix:arbory_file_remove_and_add_fix into a1b7b3d on arbory:master.