Remove va not None check

Instead of checking if `va` is `None in `get_section()` we should avoid
calling this function with `None`. This have been fixed in the following
PR, so this is not longer needed:
https://github.com/fireeye/capa/pull/442

capa/features/extractors/viv/insn.py

@@ -488,7 +488,7 @@ def extract_insn_segment_access_features(f, bb, insn):
 
 def get_section(vw, va):
     for start, length, _, __ in vw.getMemoryMaps():
-        if va and start <= va < start + length:
+        if start <= va < start + length:
             return start
 
     raise KeyError(va)