set_flags_from_control: remove function

set_flags_from_control: remove function

During installs, install_data_files calls set_flags_from_control to
capture the case where fields, such as "Essential", are present in the
package control file but are missing on the Packages file. This
operation is expensive (re-parses the control file) and is catering to a
case that should not be supported on the first place.

Signed-off-by: Alejandro del Castillo <alejandro.delcastillo@ni.com>
[Jo-Philipp Wich: remove function prototype as well]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>