🐛 ppn2ocr: Verify oai.sbb.berlin's certificate again

Now that oai.sbb.berlin's certificate chain is fixed, remove the
workaround again.

Fixes GH#15.

ppn2ocr

@@ -32,9 +32,7 @@ def oai_mets(ppn):
     }
 
     s = requests.Session()
-    # FIXME oai.sbb.berlin fails certificate check
-    #r = s.get(API_URL, params=params)
-    r = s.get(API_URL, params=params, verify=False)
+    r = s.get(API_URL, params=params)
     mets = ET.XML(r.content).find(f".//{{{XMLNS['mets']}}}mets")
     mets = ET.ElementTree(mets)