Do not ensure consistency of "Section" field across unstable and testing

In Debian, the same override is applied to both suites and they are
always consistent. It's not the case in Kali, we keep the value from
Debian when we import the package in britney's source distribution, but
if the same version is already present in the target distribution, it
keeps its original section (dating back to its initial import). In those
situations, the code will fail with an error like this one:

E: [2018-12-28T19:57:57+0000] - Mismatch found coinor-libdylp0 1.6.0-1.1 amd64 differs
I: [2018-12-28T19:57:57+0000] -  ... section libs != science
[...]
ValueError: Inconsistent / Unsupported data set

Commit 7efa865a04892b94a0ef18b2e611f210a6963925 which was supposed to
move code around introduced the check on this field. Prior to this, the
section was not checked. Since the section only has an impact on which
packages take part to the smooth updates feature, the impact of such a
mismatch is negligible so I simply dropped that check.

britney2/inputs/suiteloader.py

@@ -71,7 +71,6 @@ class SuiteContentLoader(object):
 class DebMirrorLikeSuiteContentLoader(SuiteContentLoader):
 
     CHECK_FIELDS = [
-        'section',
         'source',
         'source_version',
         'architecture',